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MIC2193
Micrel
MIC2193
400kHz SO-8 Synchronous Buck Control IC Final Information
General Description
Micrel's MIC2193 is a high efficiency PWM synchronous buck control IC housed in the SO-8 package. Its 2.9V to 14V input voltage range allows it to efficiently step down voltages in 3.3V, 5V, and 12V systems as well as 1- or 2-cell Li Ion battery powered applications. The MIC2193 solution saves valuable board space. The device is housed in the space-saving SO-8 package, whose low pin-count minimizes external components. Its 400kHz PWM operation allows a small inductor and small output capacitors to be used. The MIC2193 can implement allceramic capacitor solutions. The MIC2193 drives a high-side P-channel MOSFET, eliminating the need for high-side boot-strap circuitry. This feature allows the MIC2193 to achieve maximum duty cycles of 100%, which can be useful in low headroom applications. A low output driver impedance of 4 allows the MIC2193 to drive large external MOSFETs to generate a wide range of output currents. The MIC2193 is available in an 8 pin SOIC package with a junction temperature range of -40C to +125C.
Features
* * * * * * * * * * * * * * * * 2.9V to 14V input voltage range 400kHz oscillator frequency PWM current mode control 100% maximum duty cycle Front edge blanking 4 output drivers Cycle-by-cycle current limiting Frequency foldback short circuit protection 8 lead SOIC package Point of load power supplies Distributed power systems Wireless Modems ADSL line cards Servers Step down conversion in 3.3V, 5V, 12V systems 1-and 2-cell Li Ion battery operated equipment
Applications
Typical Application
VIN 3.3V 120F 6.3V (x2) 2k 1F 2.2nF
0.012 MIC2193BM VIN VDD CS OUTP Si9803 (x2) 3.8H Si9804 (x2) 10k 22.6k
VOUT 1.8V, 5A 220F 6.3V (x2)
COMP OUTN GND FB
Adjustable Output Synchronous Buck Converter
Micrel, Inc. * 1849 Fortune Drive * San Jose, CA 95131 * USA * tel + 1 (408) 944-0800 * fax + 1 (408) 944-0970 * http://www.micrel.com
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MIC2193
MIC2193
Micrel
Ordering Information
Part Number MIC2193BM Output Voltage Adjustable Frequency 400KHz Junction Temp. Range -40C to +125C Package 8-lead SOP
Pin Configuration
VIN 1 COMP 2 FB 3 CS 4 8 OUTP 7 OUTN 6 GND 5 VDD
8 Lead SOIC (M)
Pin Description
Pin Number 1 2 3 4 Pin Name VIN COMP FB CS Pin Function Controller supply voltage. Also the (+) input to the current sense amp. Compensation (Output): Internal error amplifier output. Connect to a capacitor or series RC network to compensate the regulator's control loop. Feedback Input: The circuit regulates this pin to 1.245V. The (-) input to the current limit comparator. A built in offset of 110mV between VIN and CSL in conjunction with the current sense resistor sets the current limit threshold level. This is also the (-) input to the current amplifier. 3V internal linear-regulator output. VDD is also the supply voltage bus for the chip. Bypass to GND with 1F. Ground High current drive for the synchronous N-channel MOSFET. Voltage swing is from ground to VIN. On-resistance is typically 6 at 5VIN. High current drive for the high side P-channel MOSFET. Voltage swing is from ground to VIN. On-resistance is typically 6 at 5VIN.
5 6 7 8
VDD GND OUTN OUTP
MIC2193
2
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MIC2193
Micrel
Absolute Maximum Ratings (Note 1)
Supply Voltage (VIN) ..................................................... 15V Digital Supply Voltage (VDD) ........................................... 7V Comp Pin Voltage (VCOMP) ............................ -0.3V to +3V Feedback Pin Voltage (VFB) .......................... -0.3V to +3V Current Sense Voltage (VIN - VCS) ................ -0.3V to +1V Power Dissipation (PD) ..................... 285mW @ TA = 85C Ambient Storage Temp ............................ -65C to +150C ESD Rating Note 3 ....................................................... 2kV
Operating Ratings (Note 2)
Supply Voltage (VIN) .................................... +2.9V to +14V Junction Temperature ....................... -40C TJ +125C Package Thermal Resistance JA 8-lead SOP ................................................. 140C/W
Electrical Characteristics
VIN = 5V, VOUT = 3.3V, TJ = 25C, unless otherwise specified. Bold values indicate -40CMarch 2002
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MIC2193
Parameter Gate Drivers Rise/Fall Time Output Driver Impedance CL = 3300pF Source, VIN = 12V Sink, VIN = 12V Source, VIN = 5V Sink, VIN = 5V VIN = 12V VIN = 5V VIN = 3.3V 50 4 4 6 6 50 80 160 10 10 12 12 Condition Min Typ Max
Micrel
Units
ns ns ns ns
Driver Non-overlap Time
Note 1.
Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(Max), the junction-to-ambient thermal resistance, JA, and the ambient temperature, TA. The device is not guaranteed to function outside its operating rating. Devices are ESD sensitive, handling precautions required. Human body model, 1.5k in series with 100pF.
Note 2. Note 3.
MIC2193
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MIC2193
Micrel
Typical Characteristics
Quiescent Current vs. Supply Voltage
6 QUIESCENT CURRENT (mA) QUIESCENT CURRENT (mA) 5 4 3 2 1 0 0 5 10 SUPPLY VOLTAGE (V) 15 2.0 1.8 1.6 VDD (V) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 VIN = 5V 0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
Quiescent Current vs. Temperature
3.15 3.10 3.05 3.00 2.95 2.90 2.85 2.80 0
VDD vs. Input Voltage
5 10 INPUT VOLTAGE (V)
15
3.10 3.08 3.06 VDD (V) 3.04 3.02 3.00 2.98 2.96 2.94 2.92 2.90 0
VDD vs. Load
VIN = 12V VDD (V)
3.50 3.40 3.30 3.20 3.10 3.00 2.90 2.80 2.70
VDD vs. Temperature
REFERENCE VOLTAGE (V)
Reference Voltage vs. Temperature
1.300 1.290 1.280 1.270 1.260 1.250 1.240 1.230 1.220 1.210 1.200 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C) VIN = 5V
VIN = 5V VIN = 3.3V
0.2 0.4 0.6 0.8 1 1.2 VDD LOAD CURRENT (mA)
2.60 VIN = 5V 2.50 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
CURRENT LIMIT THRESHOLD (mV)
Switching Frequency vs. Input Voltage
2.5 FREQUENCY VARIATION (%) 2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 0 5 10 INPUT VOLTAGE (V) 15 FREQUENCY VARIATION (%) 5 0 -5 -10 -15
Switching Frequency vs. Temperature
Overcurrent Threshold vs. Input Voltage
130 125 120 115 110 105 100 95 90 0 2 4 6 8 10 12 14 INPUT VOLTAGE (V)
VIN = 5V -20 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
Current Limit Threshold vs. Temperature
CURRENT LIMIT THREHOLD (mV) 120 115 110 105 100 95 90 85 80 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C) VIN = 5V IMPEDANCE () 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 0
OUTN Drive Impedance vs. Input Voltage
14 12 IMPEDANCE () 10 8 6 4
OUTN Drive Impedance vs. Input Voltage
Sink ()
Source ()
Source () Sink () 2 4 6 8 10 12 14 INPUT VOLTAGE (V) 2 0 0 5 10 INPUT VOLTAGE (V) 15
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Functional Diagram
VIN CIN CDECOUP
VIN
1
OVERCURRENT COMPARATOR
VREF 1.245V
RSENSE VDD BIAS
5 VDD GAIN 3 4
CSL
ON
CURRENT SENSE AMP
VIN
fs/4 CONTROL
8
OUTP Q1 L1 VOUT
7
OUTN
Q2
D1
OSC
COUT
RESET SLOPE COMPENSATION
PWM COMPARATOR gm = 0.0002 VREF gain = 20 COMP
2
ERROR AMP 100k
3
FB
0.3V fs/4
FREQUENCY FOLDBACK
6
GND
Figure 1. MIC2193 Block Diagram
Functional Characteristics
Controller Overview and Functional Description The MIC2193 is a BiCMOS, switched mode, synchronous, step down (buck) converter controller. It uses both N- and Pchannel MOSFETs, which allows the controller to operate at 100% duty cycle and eliminates the need for a high side drive bootstrap circuit. Current mode control is used to achieve superior transient line and load regulation. An internal corrective ramp provides slope compensation for stable operation above a 50% duty cycle. The controller is optimized for high efficiency, high performance DC-DC converter applications. Figure 1 is a block diagram of the MIC2193 configured as a synchronous buck converter. At the beginning of the switchMIC2193 6
ing cycle, the OUTP pin pulls low and turns on the high-side P-Channel MOSFET, Q1. Current flows from the input to the output through the current sense resistor, MOSFET and inductor. The current amplitude increases, controlled by the inductor. The voltage developed across the current sense resistor, RSENSE, is amplified inside the MIC2193 and combined with an internal ramp for stability. This signal is compared to the output of the error amplifier. When the current signal equals the error voltage signal, the P-channel MOSFET is turned off. The inductor current flows through the diode, D1, until the synchronous, N-channel MOSFET turns on. The voltage drop across the MOSFET is less than the forward voltage drop of the diode, which improves the converter efficiency. At the end of the switching period, the synchronous MOSFET is turned off and the switching cycle repeats. March 2002
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The MIC2193 controller is broken down into five functions. * Control loop - PWM operation - Current mode control * Current limit * Reference and VDD * MOSFET gate drive * Oscillator Control Loop
PWM Control Loop
Micrel
Current Limit The output current is detected by the voltage drop across the external current sense resistor (RSENSE in Figure 1.). The current sense resistor must be sized using the minimum current limit threshold. The external components must be designed to withstand the maximum current limit. The current sense resistor value is calculated by the equation below:
RSENSE = MIN _ CURRENT _ SENSE _ THRESHOLD IOUT _ MAX
The MIC2193 uses current mode control to regulate the output voltage. This dual control loop method (illustrated in Figure 2) senses the output voltage (outer loop) and the inductor current (inner loop). It uses inductor current and output voltage to determine the duty cycle of the buck converter. Sampling the inductor current effectively removes the inductor from the control loop, which simplifies compensation.
VIN Switching Converter VOUT
The maximum output current is:
IOUT _ MAX = MAX _ CURRENT _ SENSE _ THRESHOLD RSENSE
Voltage Divider IINDUCTOR Switch Driver VERROR VREF
IINDUCTOR
VERROR
tON tPER D = tON/tPER
Figure 2. Current Mode Control Example As shown in Figure 1, the inductor current is sensed by measuring the voltage across the resistor, RSENSE. A ramp is added to the amplified current sense signal to provide slope compensation, which is required to prevent unstable operation at duty cycles greater than 50%. A transconductance amplifier is used for the error amplifier, which compares an attenuated sample of the output voltage with a reference voltage. The output of the error amplifier is the compensation pin (COMP), which is compared to the current sense waveform in the PWM block. When the current signal becomes greater than the error signal, the comparator turns off the high-side drive. The COMP pin provides access to the output of the error amplifier and allows the use of external components to stabilize the voltage loop.
The current sense pins VIN (pin 1) and CSL (pin 4) are noise sensitive due to the low signal level and high input impedance and switching noise on the VIN pin. The PCB traces should be short and routed close to each other. A 10nF capacitor across the pins will attenuate high frequency switching noise. When the peak inductor current exceeds the current limit threshold, the overcurrent comparator turns off the high side MOSFET for the remainder of the switching cycle, effectively decreasing the duty cycle. The output voltage drops as additional load current is pulled from the converter. When the voltage at the feedback pin (FB) reaches approximately 0.3V, the circuit enters frequency foldback mode and the oscillator frequency will drop to approximately 1/4 of the switching frequency. This limits the maximum output power delivered to the load under a short circuit condition. Reference and VDD Circuits The output drivers are enabled when the VDD voltage (pin 5) is greater than its undervoltage threshold. The internal bias circuit generates an internal 1.245V bandgap reference voltage for the voltage error amplifier and a 3V VDD voltage for the internal control circuitry. The VDD pin must be decoupled with a 1F ceramic capacitor. The capacitor must be placed close to the VDD pin. The other end of the capacitor must be connected directly to the ground plane. MOSFET Gate Drive The MIC2193 is designed to drive a high-side P-channel MOSFET and a low side N-channel MOSFET. The source pin of the P-channel MOSFET is connected to the input of the power supply. It is turned on when OUTP pulls the gate of the MOSFET low. The advantage of using a P-channel MOSFET is that it does not required a bootstrap circuit to boost the gate voltage higher than the input, as would be required for an Nchannel MOSFET. The VIN pin (pin 1) supplies the drive voltage to both gate drive pins, OUTN and OUTP. The VIN pin must be well decoupled to prevent noise from affecting the current sense circuit, which uses VIN as one of the sense pins. A non-overlap time is built into the MOSFET driver circuitry. This dead time prevents the high-side and low-side MOSFET drivers from being on at the same time. Either an external diode or the low-side MOSFET internal parasitic diode conducts the inductor current during the dead time.
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MIC2193
MIC2193
MOSFET Selection The P-channel MOSFET must have a VGS threshold voltage equal to or lower than the input voltage when used in a buck converter topology. There is a limit to the maximum gate charge the MIC2193 will drive. MOSFETs with higher gate charge will have slower turn-on and turn-off times. Slower transition times will cause higher power dissipation in the MOSFETs due to higher switching transition losses. The MOSFETs must be able to completely turn on and off within the driver non-overlap time If both MOSFETs are conducting at the same time, shoot-through will occur, which greatly increases power dissipation in the MOSFETs and reduces converter efficiency. The MOSFET gate charge is also limited by power dissipation in the MIC2193. The power dissipated by the gate drive circuitry is calculated below: PGATE_DRIVE = QGATE x VIN x fS where: QGATE is the total gate charge of both the N and Pchannel MOSFETs. fS is the switching frequency VIN is the gate drive voltage The graph in Figure 3 shows the total gate charge that can be driven by the MIC2193 over the input voltage range, for different values of switching frequency.
Max. Gate Charge
MAXIMUM GATE CHARGE (nC)
Micrel
MIC2193 Voltage Amplifier Pin 3 R2 VREF 1.245V VOUT
R1
Figure 4 The output voltage is determined by the equation below. R1 R2 Where: VREF for the MIC2193 is typically 1.245V. Lower values of R1 are preferred to prevent noise from appearing on the FB pin. A typically recommended value is 10k. If R1 is too small in value it will decrease the efficiency of the power supply, especially at low output loads. Once R1 is selected, R2 can be calculated with the following formula. VOUT = VREF x 1 + R2= VREF x R1 VOUT - VREF
100 90 80 70 60 50 40 30 20 10 0 0
2
4 6 8 10 12 14 INPUT VOLTAGE (V)
Figure 3. MIC2193 Frequency vs Max. Gate Charge Oscillator The internal oscillator is free running and requires no external components. The maximum duty cycle is 100%. This is another advantage of using a P-channel MOSFET for the high-side drive; it can continuously turned on. A frequency foldback mode is enabled if the voltage on the feedback pin (pin 3) is less than 0.3V. In frequency foldback, the oscillator frequency is reduced by approximately a factor of 4. Frequency foldback is used to limit the energy delivered to the output during a short circuit fault condition. Voltage Setting Components The MIC2193 requires two resistors to set the output voltage as shown in Figure 4.
Efficiency Considerations Efficiency is the ratio of output power to input power. The difference is dissipated as heat in the buck converter. Under light output load, the significant contributors are: * The VIN supply current To maximize efficiency at light loads: * Use a low gate charge MOSFET or use the smallest MOSFET, which is still adequate for maximum output current. * Use a ferrite material for the inductor core, which has less core loss than an MPP or iron power core. Under heavy output loads the significant contributors to power loss are (in approximate order of magnitude): * Resistive on time losses in the MOSFETs * Switching transition losses in the high side MOSFET * Inductor resistive losses * Current sense resistor losses * Input capacitor resistive losses (due to the capacitors ESR) To minimize power loss under heavy loads: * Use low on resistance MOSFETs. Use low threshold logic level MOSFETs when the input voltage is below 5V. Multiplying the gate charge by the on resistance gives a figure of merit, providing a good balance between low load and high load efficiency. * Slow transition times and oscillations on the voltage and current waveforms dissipate more power during the turn on and turn off of the MOSFETs. A clean layout will minimize parasitic inductance and capaci tance in the gate drive and high current paths. This will allow the fastest transition times and waveforms without oscillations. Low gate charge MOSFETs will 8 March 2002
MIC2193
MIC2193
Micrel
Package Information
0.026 (0.65) MAX) PIN 1
0.157 (3.99) 0.150 (3.81)
DIMENSIONS: INCHES (MM)
0.050 (1.27) TYP
0.020 (0.51) 0.013 (0.33) 0.0098 (0.249) 0.0040 (0.102) 0-8 SEATING PLANE 45 0.010 (0.25) 0.007 (0.18)
0.064 (1.63) 0.045 (1.14)
0.197 (5.0) 0.189 (4.8)
0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.228 (5.79)
8-Pin SOIC (M)
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
USA
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. (c) 2002 Micrel Incorporated
March 2002
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MIC2193

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