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L6911D
5 BIT PROGRAMMABLE STEP DOWN CONTROLLER WITH SYNCHRONOUS RECTIFICATION
s s s
s s
s s s s s s s
OPERATING SUPPLY IC VOLTAGE FROM 5V TO 12V BUSES UP TO 1.3A GATE CURRENT CAPABILITY TTL-COMPATIBLE 5 BIT PROGRAMMABLE OUTPUT COMPLIANT WITH VRM 9.0 : 1.100V TO 1.850V WITH 0.025V BINARY STEPS VOLTAGE MODE PWM CONTROL EXCELLENT OUTPUT ACCURACY: 1% OVER LINE AND TEMPERATURE VARIATIONS VERY FAST LOAD TRANSIENT RESPONSE: FROM 0% TO 100% DUTY CYCLE POWER GOOD OUTPUT VOLTAGE OVERVOLTAGE PROTECTION AND MONITOR OVERCURRENT PROTECTION REALIZED USING THE UPPER MOSFET'S RdsON 200KHz INTERNAL OSCILLATOR OSCILLATOR EXTERNALLY ADJUSTABLE FROM 50KHz TO 1MHz SOFT START AND INHIBIT FUNCTIONS
SO-20 ORDERING NUMBERS: L6911D L6911DTR (Tape and Reel)
APPLICATIONS s POWER SUPPLY FOR ADVANCED MICROPROCESSOR CORE s DISTRIBUTED POWER SUPPLY s HIGH POWER DC-DC REGULATORS
DESCRIPTION The device is a power supply controller specifically designed to provide a high performance DC/DC conversion for high current microprocessors. A precise 5-bit digital to analog converter (DAC) allows adjusting the output voltage from 1.30V to 2.05V with 50mV binary steps and from 2.10V to 3.50V with 100mV binary steps. The high precision internal reference assures the selected output voltage to be within 1%. The high peak current gate drive affords to have fast switching to the external power mos providing low switching losses. The device assures a fast protection against load overcurrent and load overvoltage. An external SCR is triggered to crowbar the input supply in case of hard over-voltage. An internal crowbar is also provided turning on the low side mosfet as long as the overvoltage is detected. In case of over-current detection, the soft start capacitor is discharged and the system works in HICCUP mode.
BLOCK DIAGRAM
Vcc 5 to 12V Vin 5V to12V
PGOOD SS
VCC
OCSET BOOT
MONITOR and PROTECTION OVP RT
UGATE
OSC
PHASE LGATE PGND
Vo 1.100V to 1.850V
VD0 VD1 VD2 VD3 VD4 D/A + E/A + PWM
GND VSEN VFB
D98IN957_2
COMP
November 2001
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L6911D
ABSOLUTE MAXIMUM RATINGS
Symbol VCC VBOOT-VPHASE VHGATE-VPHASE OCSET, LGATE, PHASE RT, SS, FB, PGOOD, VSEN, VID0-4 OVP, COMP VCC to GND, PGND Boot Voltage Parameter Value 15 15 15 -0.3 to Vcc+0.3 7 6.5 Unit V V V V V V
THERMAL DATA
Symbol Rth j-amb Tj Tstg TJ Parameter Thermal Resistance Junction to Ambient Maximum junction temperature Storage temperature range Junction temperature range Value 110 150 -40 to 150 0 to 125 Unit C/W C C C
PIN CONNECTION (Top view)
VSEN OCSET SS/INH VID0 VID1 VID2 VID3 VID4 COMP FB
1 2 3 4 5 6 7 8 9 10
D98IN958
20 19 18 17 16 15 14 13 12 11
RT OVP VCC LGATE PGND BOOT UGATE PHASE PGOOD GND
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PIN FUNCTION
Pin Num. 1 2 Name VSEN OCSET Description Connected to the output voltage is able to manage over-voltage conditions and the PGOOD signal. A resistor connected from this pin and the upper Mos Drain sets the current limit protection. The internal 200A current generator sinks a current from the drain through the external resistor. The Over-Current threshold is due to the following equation: I OCSE T R O CS ET IP = --------------------------------------------R DS on The soft start time is programmed connecting an external capacitor from this pin and GND. The internal current generator forces through the capacitor 10A. This pin can be used to disable the device forcing a voltage lower than 0.4V Voltage Identification Code pins. These input are internally pulled-up and TTL compatible. They are used to program the output voltage as specified in Table 1 and to set the overvoltage and power good thresholds. Connect to GND to program a `0' while leave floating to program a `1'. This pin is connected to the error amplifier output and is used to compensate the voltage control feedback loop. This pin is connected to the error amplifier inverting input and is used to compensate the voltage control feedback loop. All the internal references are referred to this pin. Connect it to the PCB signal ground. This pin is an open collector output and is pulled low if the output voltage is not within the above specified thresholds. If not used may be left floating. This pin is connected to the source of the upper mosfet and provides the return path for the high side driver. This pin monitors the drop across the upper mosfet for the current limit High side gate driver output. Bootstrap capacitor pin. Through this pin is supplied the high side driver and the upper mosfet. Connect through a capacitor to the PHASE pin and through a diode to Vcc (cathode vs. boot). Power ground pin. This pin has to be connected closely to the low side mosfet source in order to reduce the noise injection into the device This pin is the lower mosfet gate driver output Device supply voltage. The operative nominal supply voltage ranges from 5 to 12V. DO NOT CONNECT VIN TO A VOLTAGE GREATER THAN VCC. Over voltage protection. If the output voltage reaches the 17% above the programmed voltage this pin is driven high and can be used to drive an external SCR that crowbar the supply voltage. If not used, it may be left floating. Oscillator switching frequency pin. Connecting an external resistor from this pin to GND, the external frequency is increased according to the equation: 4.94 10 f S = 200kHz + ------------------------R T ( k ) Connecting a resistor from this pin to Vcc (12V), the switching frequency is reduced according to the equation: 4.306 10 f S = 200kHz - ---------------------------RT ( k ) If the pin is not connected, the switching frequency is 200KHz. The voltage at this pin is fixed at 1.23V (typ). Forcing a 50A current into this pin, the built in oscillator stops to switch.
7 6
3
SS/INH
4-8
VID0 - 4
9 10 11 12
COMP FB GND PGOOD
13 14 15 16 17 18 19
PHASE UGATE BOOT PGND LGATE VCC OVP
20
RT
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ELECTRICAL CHARACTERISTCS (VCC = 12V, Tamb = 25C unless otherwise specified)
Symbol Parameter Test Condition Min. Typ. Max. Unit VCC SUPPLY CURRENT Icc Vcc Supply current Turn-On Vcc threshold Turn-Off Vcc threshold Rising VOCSET threshold ISS Soft start Current UGATE and LGATE open VOCSET=4.5V VOCSET=4.5V 3.6 1.24 10 5 4.6 mA V V V A 220 15 1.9 KHz % Vp-p POWER-ON
OSCILLATOR Free running frequency Total Variation Vosc Ramp amplitude RT = OPEN 6 K < RT to GND < 200 K RT = OPEN VID0, VID1, VID2, VID3, VID4 see Table1; Tamb = 0 to 70C -1 4 88 10 COMP=10pF 1 10 1.3 2 0.9 1.1 1.5 120 117 170 60 200 120 230 3 4 180 -15 200
REFERENCE AND DAC DACOUT Voltage Accuracy VID Pull-Up voltage ERROR AMPLIFIER DC Gain GBWP SR Gain-Bandwidth Product Slew-Rate High Side Source Current High Side Sink Resistance Low Side Source Current Low Side Sink Resistance Output Driver Dead Time PROTECTIONS Over Voltage Trip (VSEN/DACOUT) IOCSET IOVP OCSET Current Source OVP Sourcing Current VSEN Rising VOCSET = 4.5V VSEN > OVP Trip, VOVP=0V VSEN Rising VSEN Falling Upper and Lower threshold IPGOOD = -5mA % A mA dB MHz V/S A A ns 1 % V
GATE DRIVERS IUGATE RUGATE ILGATE RLGATE VBOOT - VPHASE=12V, VUGATE - VPHASE= 6V VBOOT-VPHASE=12V, IUGATE = 300mA Vcc=12V, VLGATE = 6V Vcc=12V, ILGATE = 300mA PHASE connected to GND
POWER GOOD Upper Threshold (VSEN/DACOUT) Lower Threshold (VSEN/DACOUT) Hysteresis (VSEN/DACOUT) VPGOOD PGOOD Voltage Low 110 86 112 88 2 0.5 114 90 % % % V
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Table 1. VID Settings
VID4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 VID3 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 VID2 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 VID1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 VID0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 Output Voltage (V) Output OFF 1.100 1.125 1.150 1.175 1.200 1.225 1.250 1.275 1.300 1.325 1.350 1.375 1.400 1.425 1.450 VID4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 VID3 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 VID2 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 VID1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 VID0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 Output Voltage (V) 1.475 1.500 1.525 1.550 1.575 1.600 1.625 1.650 1.675 1.700 1.725 1.750 1.775 1.800 1.825 1.850
Device Description The device is an integrated circuit realized in BCD technology. It provides complete control logic and protections for a high performance step-down DC-DC converter optimized for microprocessor power supply. It is designed to drive N-Channel Mosfets in a synchronous-rectified buck topology. The device works properly with Vcc ranging from 5V to 12V and regulates the output voltage starting from a 1.26V power stage supply voltage (Vin). The output voltage of the converter can be precisely regulated, programming the VID pins, from 1.100V to 1.850V with 25mV binary steps, with a maximum tolerance of 1% over temperature and line voltage variations. The device provides voltage-mode control with fast transient response. It includes a 200kHz free-running oscillator that is adjustable from 50kHz to 1MHz. The error amplifier features a 15MHz gain-bandwidth product and 10V/s slew rate which permits high converter bandwidth for fast transient performance. The resulting PWM duty cycle ranges from 0% to 100%. The device protects against over-current conditions entering in HICCUP mode. The device monitors the current by using the rDS(ON) of the upper MOSFET which eliminates the need for a current sensing resistor. The device is available in SO20 package Oscillator The switching frequency is internally fixed to 200kHz. The internal oscillator generates the triangular waveform for the PWM charging and discharging with a constant current an internal capacitor. The current delivered to the oscillator is typically 50A (Fsw=200KHz) and may be varied using an external resistor (R T) connected between RT pin and GND or VCC. Since the RT pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied proportionally to the current sunk (forced) from (into) the pin. In particular connecting it to GND the frequency is increased (current is sunk from the pin), according to the following relationship: 4.94 10 f S = 200kH z + ------------------------R T ( k )
6
Connecting RT to VCC=12V or to VCC=5V the frequency is reduced (current is forced into the pin), according to the following relationships:
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4.306 10 f S = 200kH z + ---------------------------R T ( k ) 15 10 f S = 200kH z + -------------------R T ( k ) Switching frequency variations vs. RT are reported in Fig.1.
7
7
VCC = 12V
VCC = 5V
Note that forcing a 50A current into this pin, the device stops switching because no current is delivered to the oscillator. Figure 1.
1 0 00 0
1 00 0
R e s is ta n ce [kO h m ]
10 0
10
R T to G N D R T to V C C =1 2 V R T to V C C =5 V
10
100
1000
F re q u e n cy [kH z]
Digital to Analog Converter The built-in digital to analog converter allows the adjustment of the output voltage from 1.30V to 2.05V with 50mV binary steps and from 2.10V to 3.50V with 100mV binary steps as shown in the previous table 1. The internal reference is trimmed to ensure the precision of 1%. The internal reference voltage for the regulation is programmed by the voltage identification (VID) pins. These are TTL compatible inputs of an internal DAC that is realized by means of a series of resistors providing a partition of the internal voltage reference. The VID code drives a multiplexer that selects a voltage on a precise point of the divider. The DAC output is delivered to an amplifier obtaining the VPROG voltage reference (i.e. the set-point of the error amplifier). Internal pull-ups are provided (realized with a 5A current generator); in this way, to program a logic "1" it is enough to leave the pin floating, while to program a logic "0" it is enough to short the pin to GND. The voltage identification (VID) pin configuration also sets the power-good thresholds (PGOOD) and the overvoltage protection (OVP) thresholds. The VID code "11111" disable the device (as a short on the SS pin) and no output voltage is regulated. Soft Start and Inhibit At start-up a ramp is generated charging the external capacitor CSS by means of a 10A constant current, as shown in figure 1. When the voltage across the soft start capacitor (VSS) reaches 0.5V the lower power MOS is turned on to dis-
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charge the output capacitor. As V SS reaches 1V (i.e. the oscillator triangular wave inferior limit) also the upper MOS begins to switch and the output voltage starts to increase. The VSS growing voltage initially clamps the output of the error amplifier, and consequently VOUT linearly increases, as shown in figure 2. In this phase the system works in open loop. When V SS is equal to VCOMP the clamp on the output of the error amplifier is released. In any case another clamp on the input of the error amplifier remains active, allowing to VOUT to grow with a lower slope (i.e. the slope of the VSS voltage, see figure 2). In this second phase the system works in closed loop with a growing reference. As the output voltage reaches the desired value VPROG, also the clamp on the error amplifier input is removed, and the soft start finishes. Vss increases until a maximum value of about 4V. The Soft-Start will not take place, and the relative pin is internally shorted to GND, if both VCC and OCSET pins are not above their own turn-on thresholds. During normal operation, if any under-voltage is detected on one of the two supplies, the SS pin is internally shorted to GND and so the SS capacitor is rapidly discharged. The device goes in INHIBIT state forcing SS pin below 0.4V. In this condition both external MOSFETS are kept off. Figure 2. Soft Start
V cc Turn-on threshold V cc Vin V in Turn-on threshold V ss
to G ND
1V 0.5V
LGATE
Vout
Timing Diagram
Aquisition: CH1 = PHASE; CH2 = V OUT; CH3 = PGOOD; CH4 = VSS
Driver Section The driver capability on the high and low side drivers allows using different types of power MOS (also multiple MOS to reduce the RDSON), maintaining fast switching transition. The low-side mos driver is supplied directly by Vcc while the high-side driver is supplied by the BOOT pin. Adaptative dead time control is implemented to prevent cross-conduction and allow to use several kinds of mosfets. The upper mos turn-on is avoided if the lower gate is over about 200mV while the lower mos turn-on is avoided if the PHASE pin is over about 500mV. The upper mos is in any case turned-on after 200nS from the low side turn-off. The peak current is shown for both the upper (fig. 3) and the lower (fig. 4) driver at 5V and 12V. A 4nF capacitive load has been used in these measurements. For the lower driver, the source peak current is 1.1A @ Vcc=12V and 500mA @ Vcc=5V, and the sink peak current is 1.3A @ Vcc=12V and 500mA @ Vcc=5V. Similarly, for the upper driver, the source peak current is 1.3A @ Vboot-Vphase=12V and 600mA @ VbootVphase =5V, and the sink peak current is 1.3A @ Vboot-Vphase =12V and 550mA @ Vboot-Vphase = 5V.
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Figure 3. High Side driver peak current. Vboot-Vphase=12V (left) Vboot-Vphase=5V (right)
CH1 = High Side Gate
CH4 = Gate Current
Figure 4. Low Side driver peak current. Vcc=12V (left) Vcc=5V (right)
CH1 = Low Side Gate
CH4 = Gate Current
Monitoring and Protections The output voltage is monitored by means of pin 1 (VSEN). If it is not within 12% (typ.) of the programmed value, the powergood output is forced low. The device provides overvoltage protection, when the output voltage reaches a value 17% (typ.) grater than the nominal one. If the output voltage exceeds this threshold, the OVP pin is forced high, triggering an external SCR to shuts the supply (VIN) down, and also the lower driver is turned on as long as the over-voltage is detected. To perform the overcurrent protection the device compares the drop across the high side MOS, due to the RDSON, with the voltage across the external resistor (ROCS) connected between the OCSET pin and drain of the upper MOS. Thus the overcurrent threshold (I P) can be calculated with the following relationship: I OCS R OCS I P = -------------------------------R DSON Where the typical value of IOCS is 200A. To calculate the ROCS value it must be considered the maximum RDSON (also the variation with temperature) and the minimum value of IOCS. To avoid undesirable trigger of
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overcurrent protection this relationship must be satisfied: l I P I OUT MAX + ---- = I PEA K 2 Where I is the inductance ripple current and IOUTMAX is the maximum output current. In case of output short circuit the soft start capacitor is discharged with constant current (10A typ.) and when the SS pin reaches 0.5V the soft start phase is restarted. During the soft start the over-current protection is always active and if such kind of event occurs, the device turns off both mosfets, and the SS capacitor is discharged again (after reaching the upper threshold of about 4V). The system is now working in HICCUP mode, as shown in figure 5a. After removing the cause of the over-current, the device restart working normally without power supplies turn off and on. Figure 5.
9 8
L=1.5H, Vin=12V L=2H, Vin=12V L=3H, Vin=12V L=1.5H, Vin=5V L=2H, Vin=5V L=3H, Vin=5V
Inductor Ripple [A]
7 6 5 4 3 2 1 0 0.5 1.5 2.5
3.5
Output Voltage [V ]
a: Hiccup Mode
b: Inductor Ripple Current vs. Vout
Inductor design The inductance value is defined by a compromise between the transient response time, the efficiency, the cost and the size. The inductor has to be calculated to sustain the output and the input voltage variation to maintain the ripple current IL between 20% and 30% of the maximum output current. The inductance value can be calculated with this relationship: V IN - V OUT V OUT L = ----------------------------- -------------V IN f S I L Where fSW is the switching frequency, VIN is the input voltage and VOUT is the output voltage. Figure 5b shows the ripple current vs. the output voltage for different values of the inductor, with VIN = 5V and VIN = 12V. Increasing the value of the inductance reduces the ripple current but, at the same time, reduces the converter response time to a load transient. If the compensation network is well designed, the device is able to open or close the duty cycle up to 100% or down to 0%. The response time is now the time required by the inductor to change its current from initial to final value. Since the inductor has not finished its charging time, the output current is supplied by the output capacitors. Minimizing the response time can minimize the output capacitance required. The response time to a load transient is different for the application or the removal of the load: if during the application of the load the inductor is charged by a voltage equal to the difference between the input and the output voltage, during the removal it is discharged only by the output voltage. The following expressions give approximate response time for I load transient in case of enough fast compensation network response:
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L I t a pplic atio n = ----------------------------V IN - V OUT
L I t rem ov al = -------------V OUT
The worst condition depends on the input voltage available and the output voltage selected. Anyway the worst case is the response time after removal of the load with the minimum output voltage programmed and the maximum input voltage available. Output Capacitor Since the microprocessors require a current variation beyond 10A doing load transients, with a slope in the range of tenth A/sec, the output capacitor is a basic component for the fast response of the power supply. In fact for first few microseconds they supply the current to the load. The controller recognizes immediately the load transient and sets the duty cycle at 100%, but the current slope is limited by the inductor value. The output voltage has a first drop due to the current variation inside the capacitor (neglecting the effect of the ESL): VOUT = IOUT * ESR A minimum capacitor value is required to sustain the current during the load transient without discharge it. The voltage drop due to the output capacitor discharge is given by the following equation: I OUT L V OUT = --------------------------------------------------------------------------------------------2 C OUT ( V INM IN D M AX - V OUT ) Where DMAX is the maximum duty cycle value that is 100%. The lower is the ESR, the lower is the output drop during load transient and the lower is the output voltage static ripple. Input Capacitor The input capacitor has to sustain the ripple current produced during the on time of the upper MOS, so it must have a low ESR to minimize the losses. The rms value of this ripple is: I rm s = I OUT D ( 1 - D ) Where D is the duty cycle. The equation reaches its maximum value with D=0.5. The losses in worst case are: P = ESR I rm s Compensation network design The control loop is a voltage mode (figure 7) that uses a droop function to satisfy the requirements for a VRM module, reducing the size and the cost of the output capacitor. This method "recovers" part of the drop due to the output capacitor ESR in the load transient, introducing a dependence of the output voltage on the load current: at light load the output voltage will be higher than the nominal level, while at high load the output voltage will be lower than the nominal value.
2 2
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Figure 6. Output transient response without (a) and with (b) the droop function
ESR DROP ESR DROP
VMAX VDROOP
VNOM
VMIN
(a)
(b)
As shown in figure 6, the ESR drop is present in any case, but using the droop function the total deviation of the output voltage is minimized. In practice the droop function introduces a static error (Vdroop in figure 6) proportional to the output current. Since a sense resistor is not present, the output DC current is measured by using the intrinsic resistance of the inductance (a few m). So the low-pass filtered inductor voltage (that is the inductor current) is added to the feedback signal, implementing the droop function in a simple way. Referring to the schematic in figure 7, the static characteristic of the closed loop system is: R3 + R 8 // R9 R L R8 // R9 V OUT = V PRO G + V PRO G ------------------------------------- - ---------------------------------- I OUT R8 R2 Where VPROG is the output voltage of the digital to analog converter (i.e. the set point) and R L is the inductance resistance. The second term of the equation allows a positive offset at zero load (V+); the third term introduces the droop effect (VDROOP). Note that the droop effect is equal the ESR drop if: R L R 8 // R9 ---------------------------------- = ESR R8 Figure 7. Compensation network
V
IN
V
COMP
V
PHASE
L2
R
L
V
O UT
PW M
ESR R8
C18
Z
F
C 6-15 C 20 R4 R9
R3
C 25
V
PROG
ZI
R2
Considering the previous relationships R2, R3, R8 and R9 may be determined in order to obtain the desired droop effect as follow: s Choose a value for R2 in the range of hundreds of K to obtain realistic values for the other components.
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s
From the above equations, it results:
+ V R 2 R L I M AX R8 = ----------------------- -------------------------- ; V PRO G V DROOP
V DROOP 1 R9 = R8 -------------------------- ------------------------------------ ; V DROOP R L I M AX 1 + -------------------------R L I M AX Where IMAX is the maximum output current. The component R3 must be chosen in order to obtain R3<s
Therefore, with the droop function the output voltage decreases as the load current increases, so the DC output impedance is equal to a resistance ROUT. It is easy to verify that the output voltage deviation under load transient is minimum when the output impedance is constant with frequency. To choose the other components of the compensation network, the transfer function of the voltage loop is considered. To simplify the analysis is supposed that R3 << Rd, where Rd = (R8//R9). Figure 8. Compensation network definition
|A v |
2
fLC |R |
fC E
fEC
fC C
f
R0
fD |G lo o p | G0
f2
f1
f3
f
fc f
ConverterS ingularity
fLC = 1 / 2
LC
doublepole ESRzero Introduced by CeramicCapacitor
Compensati onNetworkS ingularity
fCE = 1 / 2 ESR C OUT = 1 / 2 ESR Cceramic f EC = 1 / 2 Rceramic Cceramic f CC
f1 = 1 / 2 R 4 C 20 f2 = 1 / 2 ( R 3 + R 4 ) C 20 f3 = 1 / 2 R 3 C 25 fd = 1 / 2 Rd C 25
The transfer function may be evaluated neglecting the connection of R8 to PHASE because, as will see later, this connection is important only at low frequencies. So R4 is considered connected to VOUT. Under this assumption, the voltage loop has the following transfer function:
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ZC ( s ) Zf ( s ) Vin G loo p ( s ) = Av ( s ) R ( s ) = Av ( s ) ------------- Where Av ( s ) = --------------- -----------------------------------V o sc Z C ( s ) + Z L ( s ) Zi ( s ) Where ZC(s) and ZL(s) are the output capacitor and inductor impedance respectively. The expression of ZI(s) may be simplified as follow: 2 R3 1 1 Rd 1 + s ( 1 + d ) + s ------- 1 d Rd -- C 25 R4 + -- C20 R 3 Rd s s Z I ( s ) = --------------------------------- + ----------------------------------------------------- = -------------------------------------------------------------------------------------------------- = (1 + s 2 ) (1 + s d ) 1 Rd + -- C25 R 4 + 1 C20 + R3 -s s 1 + s R3 ( 1 + s ) ------- d 1 Rd = R d -------------------------------------------------------------------( 1 + s 2) ( 1 + s d ) Where: 1 = R4xC20, 2 = (R4+R3)xC20 and d = RdxC25. The regulator transfer function became now: ( 1 + s 2 ) ( 1 + s d ) R ( s ) ------------------------------------------------------------------------------------------------------R3 s C 18 R d 1 + s ------- d ( 1 + s 1 ) Rd Figure 8 shows a method to select the regulator components (please note that the frequencies f EC and fCC corresponds to the singularities introduced by additional ceramic capacitors in parallel to the output main electrolytic capacitor). s To obtain a flat frequency response of the output impedance, the droop time constant d has to be equal to the inductor time constant (see the note at the end of the section): L d = R d C 25 = ------ = L RL
s
L C25 = ----------------------( R L Rd )
To obtain a constant -20dB/dec Gloop(s) shape the singularity f1 and f2 are placed in proximity of fCE and fLC respectively. This implies that: f LC f2 --- = -------f CE f1 f1 = f CE f LC R4 = R3 -------- - 1 f CE C20 1 = -- R 4 f CE 2
s
To obtain a Gloop bandwidth of fC, results:
G0 f LC = 1 fC fC V IN C20 // C25 G 0 = A 0 R 0 = ----------------- ---------------------------- = ------C18 Vosc fLC C20 C25 f LC VIN C 18 = ----------------- ---------------------------- ------Vosc C20 + C25 f C
Note. To understand the reason of the previous assumption, the scheme in figure 9 must be considered. In this scheme, the inductor current has been substituted by the load current, because in the frequencies range of interest for the Droop function these current are substantially the same and it was supposed that the droop network don't represent a charge for the inductor.
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Figure 9. Voltage regulation with droop function block scheme
V com p
A v(s)
V out
R (s )
It results:
R OUT
1 + s L 1 + s d
Iout
1 + s L 1 + sL G LO O P Vo Z OUT = --------------- = R d ----------------- ---------------------------- = R OUT -----------------I LO AD 1 + s d 1 + G L O O P 1 + sd
Because in the interested range |Gloop|>>1. To obtain a flat shape, the relationship considered will naturally follow. Application Idea: 1.100V to 1.850V / 25A Figure 10 shows an application schematic for a 1.100V to 1.850V conversion with 25A of current capability. Since the device's high gate drive, more than one mosfet for both high side and low side can be used: three STS11NF30L (30V, 9mW typ @ Vgs=10V) mosfet are suggested for high side while four of them are suggested as low side switch. Figure 10. Schematic Circuit
F1 +5 VIN L1
C1-3 D1 BOOT 15 R10 +12Vcc C17 GND 11 14 VID0 VID0 VID1 VID1 VID2 VID2 VID3 VID3 VID4 VID25mV R1 OSC 20 SS 3 C16 COMP 9 10 R8 VFB C18 R3 R9 1 VSEN 8 PGOOD 12 7 16 6 5 4 PHASE 13 Q1,Q2, Q3 L2 UGATE R13 VCC 18 2 OVP 19 C24 OCSET R7 C23
C21-22
VOUTCORE LGATE R14 Q4,Q5,Q6 PGND Vss D2 C4-9 R15 R6
U1 L6911D
17
PWRGD
C19
R5
C20
R4
C17
R2
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Part List
Resistor R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R12 R13, R14 Capacitor C1-C3 C4-C9 C16 C17 C18 C19 C20 C23 C24 C25 Magnetics L1 L2 Transistors Q1-Q5 Diodes D1 D2 Ics U1 Fuse F1 251015A-15A Littlefuse AXIAL L6911D STMicroelectronics SO20 1N4148 STPS3L25U STMicroelectronics STMicroelectronics SOT23 SMB STS12NF30L STMicroelectronics SO8 1.5H 1.8H T44-52 Core, 7T-18AWG T50-52B Core, 7T-16AWG 680F - 6.3V 820F - 4V 680F - 6.3V 100n 100n 2.2n Not Mounted 100n 1n 100n 47n OSCON 6SP680M OSCON 6SP680M OSCON 4SP820M Radial 10x10.5 Radial 10x10.5 Radial 10x10.5 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 Not Mounted 470 1k 82 Not Mounted 1k 1k 13k 100k Not Mounted 20k Short Circuit 1% SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805 SMD 0805
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L6911D
mm DIM. MIN. A A1 B C D E e H h L K 10 0.25 0.4 2.35 0.1 0.33 0.23 12.6 7.4 1.27 10.65 0.75 1.27 0.394 0.010 0.016 TYP. MAX. 2.65 0.3 0.51 0.32 13 7.6 MIN. 0.093 0.004 0.013 0.009 0.496 0.291
inch TYP. MAX. 0.104 0.012 0.020 0.013 0.512 0.299 0.050 0.419 0.030 0.050
OUTLINE AND MECHANICAL DATA
SO20
0 (min.)8 (max.)
L
h x 45
A B e K H D A1 C
20
11 E
1
0 1
SO20MEC
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L6911D
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