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600kHz/200kHz Step-Down DC/DC Converter
POWER MANAGEMENT Description
The SC2612 is a voltage mode switcher designed for low cost, "point of use" voltage conversion. SC2612 is available with fixed switching frequencies of 600kHz (SC2612A) and 200kHz (SC2612C). The SC2612 has soft start and enable functions and is short circuit protected. The output of the switcher may be set anywhere between 0.8V and 75% of Vin. Short circuit protection is disabled during start-up to allow the output capacitors time to fully charge.
SC2612A/C
Features
u u u u u u
Operating frequency of 600kHz or 200kHz Input supply of 3V to 8V 0.5A Drive current for up to 10A output Output voltages down to 0.8V Overcurrent protection and soft start MSOP-8 package
Applications
u Graphics IC Power supplies u Embedded, low cost, high efficiency converters
Typical Application Circuit
12V IN 3.3V IN R1 C10 C1 C2
U2 4 1 2 C5 C7 R9 C9 R10 5 VCC BST 8 7 6 3 R2 R3 Q2 L1 1.5V OUT Q3 R6
COMP DH SS/EN GND DL FB
SC2612
C3
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SC2612A/C
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied.
Parameter Input Supply Voltage Boost Pin Voltage DL to GND
(2)
Symbol VCC VBST
(2)
Maximum 15 20 -1 to +20 0 to 70 125 -65 to 150 300 113 42 2
Units V V V C C C C C/W C/W kV
, DH to GND
VDLO, VDHI TA TJ TSTG TLEAD
(3)
Operating Ambient Temperature Range Operating Junction Temperature Storage Temperature Lead Temperature (Soldering) 10s Thermal Resistance Junction to Ambient Thermal Resistance Junction to Case ESD Rating (Human Body Model)
JA JC ESD
Electrical Characteristics
Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70C
Parameter VCC Supply Voltage VCC Quiescent Current BST Supply Voltage BST Quiescent Current VCC Under Voltage Lockout BST Under Voltage Lockout Output Voltage Overcurrent trip voltage Load Regulation Line Regulation Oscillator Frequency
Symbol VCC IQVCC VBST IQBST UVVCC UVBST VOS VITS
Conditions
Min 3.0
Typ
Max 15
Units V mA V mA V V mV V % %
VCC = 5.0V, VBST = 12.0V, SS/EN = 0V 11 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V 2.3 7.0 IO = 0; VFB = VOS, TA = 25C IO = 0.2A to 4A 792 0.4
5
10 20 5
2.6 8.0 800
2.9 9.0 808 0.7
1 0.5
fOSC
SC2612A SC2612C
480 160 80 0.3
600 200
720 240 kHz % 0.8 V A A mA A mA
Oscillator Max Duty Cycle SS/EN Shutdown Voltage SS/EN Charge current Peak DH Sink/Source Current Peak DL Sink/Source Current
MAX VSS ISS
SC2612A, SC2612C
Vss = 0.8V BST - DH = 4.5V, BST - DL = 4.5V, DH - GND = 3.3V DH - GND = 1.5V DL - GND = 3.3V DL - GND = 1.5V 0.5 50 0.5 50
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SC2612A/C
POWER MANAGEMENT Electrical Characteristics
Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70C
Parameter Error Amplifier Transconductance Error Amplifier Gain Error Amplifier Source/Sink Current Modulator Gain Dead Time
Symbol
Conditions
Min
Typ 0.8
Max
Units mS dB A dB ns
gm
A EA AM RCOMP = open V C C = 5V
45 60 19 50
Notes: (1) See Gate Resistor selection recommendations (2) 1 square inch of FR4, double sided, 1oz. minimum copper weight.
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SC2612A/C
POWER MANAGEMENT Pin Configuration
TOP VIEW
COMP SS/EN FB VCC 1 2 3 4 8 7 6 5 BST DH DL GND
Ordering Information
Part Numbers
(1)
Frequency 600kHz 600kHz 200kHz 200kHz
P ackag e
SC2612AMSTR SC2612AMSTRT (2) SC2612CMSTR SC2612CMSTRT (2)
MSOP-8
(MSOP-8)
Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free products.
Pin Descriptions
Pin # 1 2 3 4 5 6 7 8 Pin Name COMP SS/EN FB VCC GND DL DH BST Pin Function Output of the Switcher section voltage error amplifier. Soft start and enable pin, controls the switcher output voltage ramp rate. Switcher section feeedback input. Chip Supply Input Voltage. Analog and Power Ground, connect directly to ground plane, see layout guidelines. Switcher Low side FET drive output. Switcher High side FET drive output. Supply voltage for FET drives.
Block Diagram
VCC
VREF UVLO + UVLO & REF LEVEL SHIFT AND HIGH SIDE DRIVE
BST DH
SHDN +
R Q SHOOT -T HRU CONT RO L
FB COMP SS/EN
-
+
S
VREF 25uA OSCILLAT OR + + SSOVER
R Q S SYNCHRONOUS MOSFET DRIVE
DL GND
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SC2612A/C
POWER MANAGEMENT Theory of Operation
The SC2612 is a step down DC/DC controller designed for minimum cost and size without sacrificing accuracy and protection. Overcurrent protection is implemented by a simple undervoltage detection scheme and is disabled until soft start has been completed to eliminate false trips due to output capacitor charging. The SS/EN pin is held low, as are the DH and DL pins, until the undervoltage lockout points are exceeded. Once the VCC and BST pins both rise above their undervoltage lockout points, the SS capacitor begins to charge, controlling the duty cycle of the switcher, and therefore slowly ramping up the switcher output voltage. Once the SS capacitor is charged, the current limit circuitry is enabled. If a short circuit is applied , the output will be pulled down below it's trip point and shut down. The device may be restarted by either cycling power, or momentarily pulling SS/EN low.
Component Selection
OUTPUT INDUCTOR - A good starting point for output filter component selection is to choose an inductor value that will give an inductor ripple current of approximately 20% of max. output current. Inductor ripple current is given by:ae Vo VO x c1 - O / c V/ IN o e = L x fOSC
IL RIPPLE
So choose inductor value from:ae Vo 5 x VO x c1 - O / c V/ IN o e L= IO x fOSC
CAPACITOR(S) OUTPUT CAPACITOR(S) - The output capacitors should be selected to meet output ripple and transient response criteria. Output ripple voltage is caused by the inductor ripple current flowing in the output capacitor's ESR (There is also a component due to the inductor ripple current charging and discharging the output capacitor itself, but this component is usually small and can often be ignored). Given a maximum output voltage ripple requirement, ESR is given by:ae Vo VO x VRIPPLE x c1 - O / c V/ IN o e < L x fOSC
RESR
Output voltage transient excursions are a function of load current transient levels, input and output voltages and inductor and capacitor values. Capacitance and RESR values to meet a required transient condition can be calculated from:RESR < C> VT IT
2 L x IT 2 x VT x VA
where VA = VIN - VO for negative transients (load application) and VA = VO for positive transients (load release)
values for positive and negative transients must be calculated seperately and the worst case value chosen. For Capacitor values, the calculated value should be doubled to allow for duty cycle limitation and voltage drop issues.
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SC2612A/C
POWER MANAGEMENT
COMPENSATION COMPENSATION COMPONENTS - Once the filter components have been determined, the compensation components can be calculated. The goal of compensation is to modify the frequency response characteristics of the error amplifier to ensure that the closed loop feedback system has the highest gain and bandwidth possible while maintaining stability. A simplified stability criteria states that the open loop gain of the converter should fall through 0dB at 20dB/ decade at a frequency no higher than 20-25% of the switching frequency. This objective is most simply met by generating asymptotic bode plots of the small signal response of the various sections of the converter. Calculate the filter double pole frequency (Fp(lc))
Fp(lc ) = 1 2p LCo
1 2p x Co x Re sr
and calculate ESR Zero frequency (Fz(esr))
Fz( esr ) =
Choose an open loop crossover frequency (Fco) no higher than 20% of the switching frequency (Fs). The proximity of Fz(esr) to the crossover frequency Fco determines the type of compensation required, if Fz(esr)>Fco/4, use type 3 compensation, otherwise use type 2. Type 1 compensation is not appropriate and is not discussed here. Type 2 Example As an example of type 2 compensation, we will use the Evaluation board schematic.
SC2612 AND FETS
REF FB
+ EA -
MODULATOR L OUT Ra
REF + EA OUT 3.3uH VOUT 6.98k 3000uF Cs Cp 22mOhm Rs 8.06k MODULATOR
VOUT
SC2612 AND FETS
Vin=5V
COMP Zf Co Zs Zp Resr
FB
Rb
COMP
It is convenient to split the converter into two sections, the Error amp and compensation components being one section and the Modulator, output filter and divider being the other. First calculate the DC Filter+Modulator+Divider gain The DC filter gain is always 0dB, the Modulator gain is 19dB at 5V in and is proportional to Vin, so modulator gain at any input voltage is.
aeV o GMOD = 19 + 20 x Logc IN / e5o
The total Filter+Modulator+Divider DC Gain is
8.06 ae5o ae o GFMD = 19 + 20 x Logc / + 20 x Logc / = 13.6dB e5o e 6.98 + 8.06 o
This is drawn as the line A-B in Fig2
Fp(lc ) = 1 1 = 1.6kHz 2p LCo 2p 3.3 x 10 -6 x 3000 x 10 -6
1 = 2.4kHz 2p x 3000 x 10 - 6 x 22 x 10 -3
the divider gain is given by
ae R8 G DIV = 20 x Log c cR +R e5 8 o / / o
This is point B in Fig2.
Fz(esr ) =
So the total Filter+Modulator+Divider DC Gain is
ae RB o aeV o GFMD = 19 + 20 x Logc IN / + 20 x Logc / cR + R / e5o Bo eA
This is point C in Fig2., the line joining B-C slopes at 40dB/decade, the line joining C-D slopes at -20dB/decade. For 600kHz switching frequency, crossover is designed for 100kHz. Since Fz(esr)<6 www.semtech.com
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SC2612A/C
POWER MANAGEMENT
Having plotted the line ABCD, and confirmed the type of compensation necessary, compensation component values can be determined. At Fco, the line ABCD shows a gain of -27.5dB and a slope of -20dB/decade. In order for the total open loop gain to be 0dB with a -20dB/decade slope at this frequency, the compensated error amp gain at Fco must be +27.5dB with a 0dB slope. This is the line FG on the plot below. Since open loop DC gain should be as high as possible to minimize errors, a zero is placed at F and to minimize high frequency gain and switching interference a pole is placed at G. The zero at F should be no higher than Fco/4 and the pole at G no lower than 4*Fco. The equations to set the gain and the pole and zero locations are:
A
10 20 Rs = where A = gain at Fco (in dB) gm
Cs = 1 2p x Fz1 x Rs
Cp =
1 2p x Fp1 x Rs
27.5
For this example, this results in the following values.
10 20 Rs = = 29.6kW 30kW 0.8
Cs Cp
100
1 = 0.22nF 6 x 25 x 103 x 30 x 10 3 1 = 14pF (unecessar y due to EA rolloff ) 6 x 400 x 10 3 x 30 x 10 3
80
E
60
Compensated Error Amp gain F G H Fz1 C Fp1 Total open loop gain
40 Gain (dB)
20
A Fp(lc)
B
0
Fz(esr) -20 Filter+modulator +divider gain -40 Fco D
-60 100.0E+0
1.0E+3
10.0E+3 Frequency (Hz)
100.0E+3
1.0E+6
Fig2: Type 2 Error Amplifier Compensation
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SC2612A/C
POWER MANAGEMENT Layout Guidelines
Careful attention to layout requirements are necessary for successful implementation of the SC2612 PWM controller. High currents switching at high frequency are present in the application and their effect on ground plane voltage differentials must be understood and minimized. 1). The high power parts of the circuit should be laid out first. A ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. Isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom FET ground. 2). The loop formed by the Input Capacitor(s) (Cin), the Top FET (Q1) and the Bottom FET (Q2) must be kept as small as possible. This loop contains all the high current, fast transition switching. Connections should be as wide and as short as possible to minimize loop inductance. Minimizing this loop area will a) reduce EMI, b) lower ground injection currents, resulting in electrically "cleaner" grounds for the rest of the system and c) minimize source ringing, resulting in more reliable gate switching signals. 3). The connection between the junction of Q1, Q2 and the output inductor should be a wide trace or copper region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. The connection between the output inductor and the output capacitors should be a wide trace or copper area, there are no fast voltage or current transitions in this connection and length is not so important, however adding unnecessary impedance will reduce efficiency.
12V IN
Vin
10 U1 4 1 0.1uF 2 5 VCC BST 8 7 6 3 Cin Q2 L Cout 10uF Q1
COMP DH SS/EN GND DL FB
Vout
SC2612
GND
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SC2612A/C
POWER MANAGEMENT Layout Guidelines (Cont.)
4) The Output Capacitor(s) (Cout) should be located as close to the load as possible, fast transient load currents are supplied by Cout only, and connections between Cout and the load must be short, wide copper areas to minimize inductance and resistance. 5) The SC2612 is best placed over a quiet ground plane area, avoid pulse currents in the Cin, Q1, Q2 loop flowing in this area. PGNDH and PGNDL should be returned to the ground plane close to the package. The AGND pin should be connected to the ground side of (one of) the output capacitor(s). If this is not possible, the AGND pin may be connected to the ground path between the Output Capacitor(s) and the Cin, Q1, Q2 loop. Under no circumstances should AGND be returned to a ground inside the Cin, Q1, Q2 loop. 6) Vcc for the SC2612 should be supplied from the 5V supply through a 10 resistor, the Vcc pin should be decoupled directly to AGND by a 0.1F ceramic capacitor, trace lengths should be as short as possible.
Vin
Currents in Power Section
+ Vout +
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SC2612A/C
POWER MANAGEMENT Typical Characteristics
100% VBST = 12V for VIN = 5V VBST = 18V for VIN = 12V VIN = 5V Efficiency (%) 90% 12V 85% VO (V) 1.496 1.498 1.500 IO = 2.00A; VBST = 18V
95%
1.494 80% 1.492
75%
70% 0 2 4 6 8 10 Output Current (A)
1.490 4 5 6 7 8 VIN (V) 9 10 11 12
Typical Efficiency
100%
Typical Line Regulation
0.0% VBST = 12V for VIN = 5V VBST = 18V for VIN = 12V VIN = 12V
Duty Cycle (%) (No Feedback)
80%
-0.5%
60% VO (V) -1.0%
5V
40% -1.5% 20%
0% 0.0 0.2 0.4 0.6 SS/EN Voltage (V) 0.8 1.0 1.2
-2.0% 0 2 4 IO (A) 6 8 10
SS/EN Control of duty cycle
Typical Load Regulation
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SC2612A/C
POWER MANAGEMENT Evaluation Board Schematic & Layout
GND J5 12V IN J1 10 C1 0.1uF C2 10uF R1 U1 4 1 EN J11 2 5 VCC COMP SS/EN GND BST DH DL FB 8 7 6 3 Q2 Si4410DY R3 2.2 R2 2.2 Q1 Si4410DY L1 1uH R5 6.98k 1.5V OUT J4 C6 1500uF D1 EMPTY C10 R8 8.06k 1500uF
3.3V 3.3V- -12VV IN 5 IN J2 GND J6
C3 1500uF
C4 1500uF
C5 1500uF
C8 220pF
C11 EMPTY R7 30k C9 0.1uF GND J7
SC2612
GND J8
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SC2612A/C
POWER MANAGEMENT Outline Drawing - MSOP-8
Land Pattern - MSOP-8
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804
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