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CS51031
CS51031
Fast PFET Buck Controller Does Not Require Compensation
Description
The CS51031 is a switching controller for use in DC-DC converters. It can be used in the buck topology with a minimum number of external components. The CS51031 consists of a VCC monitor for controlling the state of the device, 1.0A power driver for controlling the gate of a discrete P-channel transistor, fixed frequency oscillator, short circuit protection timer, programmable soft start, precision reference, fast output voltage monitoring comparator, and output stage driver logic with latch. The high frequency oscillator allows the use of small inductors and output capacitors, minimizing PC board area and systems cost. The programmable soft start reduces current surges at start up. The short circuit protection timer significantly reduces the duty cycle to approximately 1/30 of its cycle during short circuit conditions. The CS51031 is available in 8L SO and 8L PDIP plastic packages.
Features
s 1A Totem Pole Output Driver s High Speed Oscillator (700kHz max) s No Stability Compensation Required s Lossless Short Circuit Protection s VCC Monitor s 2% Precision Reference s Programmable Soft Start
Typical Application Diagram
5V - 12V CIN 47mF 20W MP1 IRF7416
Package Options
8 Lead SO Narrow & PDIP VGATE
1
VGATE VGATE
PGnd
VC CS VCC VFB
VC
MBRS360 CS RVCC 10W CVCC 100mF CS 0.1mF D1 L 4.7mH
PGnd COSC Gnd
CS51031 COSC
COSC 150pF Gnd
VCC
VFB
100 .01mF
RB 2.5kW
VO 3.3V @ 3A
RA 1.5kW
CRR 0.1mF
CO 100mF 2
Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: info@cherry-semi.com Web Site: www.cherry-semi.com
Rev. 2/13/98
1
A
Company
CS51031
Absolute Maximum Ratings Power Supply Voltage, VCC ........................................................................................................................................................20V Driver Supply Voltage, VC ..........................................................................................................................................................20V Driver Output Voltage, VGATE ...................................................................................................................................................20V COSC, CS, VFB (Logic Pins) ............................................................................................................................................................6V Peak Output Current ................................................................................................................................................................. 1.0A Steady State Output Current ................................................................................................................................................200mA Operating Junction Temperature, TJ ..................................................................................................................................... 150C Operating Temperature Range, TA ............................................................................................................................-40 to 125C Storage Temperature Range, TS ...................................................................................................................................-65 to 150C ESD (Human Body Model).........................................................................................................................................................2kV Lead Temperature Soldering Wave Solder (through hole styles only) .....................................................................................10 sec. max, 260C peak Reflow (SMD styles only) ......................................................................................60 sec. max above 183C, 230C peak
Electrical Characteristics: Specifications apply for 4.5 VCC 16V, 3V VC 16V, -40C TJ 125C, unless otherwise specified.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
s Oscillator Frequency Charge Current Discharge Current Maximum Duty Cycle s Short Circuit Timer Charge Current Fast Discharge Current Slow Discharge Current Start Fault Inhibit Time Valid Fault Time GATE Inhibit Time Fault Duty Cycle s CS Comparator Fault Enable CS Voltage Max. CS Voltage Fault Detect Voltage Fault Inhibit Voltage Hold Off Release Voltage Regulator Threshold Voltage Clamp s VFB Comparators Regulator Threshold Voltage Fault Threshold Voltage Threshold Line Regulation Input Bias Current Voltage Tracking Input Hysteresis Voltage
VFB = 1.2V COSC = 470pF 1.4V < VCOSC < 2V 2.7V > VCOSC > 2V 1 (tOFF/tON) VFB = 1.0V; CS = 0.1F; VCOSC = 2V 1V < VCS < 2V 2.55V > VCS > 2.4V 2.4V > VCS > 1.5V 0V < VCS < 2.5V 2.6V > VCS > 2.4V 2.4V > VCS > 1.5V
160
80.0
200 110 660 83.3
240
kHz A A %
175 40 4 0.70 0.2 9 2.5
264 66 6 0.85 0.3 15 3.1
325 80 10 1.40 0.45 23 4.6
A A A ms ms ms %
VFB = 1V VFB = 1.5V VCS when GATE goes high Minimum VCS VFB = 0V VCS = 1.5V 2.5 2.6 2.4 1.5 0.7 0.866 V V V V V V
0.4 0.725
1.0 1.035
VCOSC = VCS = 2V TJ = 25C (note 1) TJ = -40 to 125C TJ = 25C (note 1) TJ = -40 to 125C 4.5V VCC 16V VFB = 0V (Regulator Threshold Voltage Fault Threshold Voltage)
1.225 1.210 1.12 1.10
1.250 1.250 1.15 1.15 6 1 100 4
1.275 1.290 1.17 1.19 15 4 120 20
V V V V mV A mV mV
70
2
CS51031
Electrical Characteristics: Specifications apply for 4.5 VCC 16V, 3V VC 16V, -40C TJ 125C, unless otherwise specified.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
s Power Stage GATE DC Low Saturation Voltage GATE DC High Saturation Voltage Rise Time Fall Time s VCC Monitor Turn On Threshold Turn Off Threshold Hysteresis s Current Drain ICC IC Shutdown ICC
VCC = VC = 10V; VFB = 1.2V VCOSC = 1V; 200mA Sink VCOSC = 2.7V; 200mA Source; VC = VGATE CGATE = 1nF; 1.5V < VGATE < 9V CGATE = 1nF; 9V > VGATE > 1.5V
1.2 1.5 25 25
1.5 2.1 60 60
V V ns ns
4.200 4.085 65
4.400 4.300 130
4.600 4.515 200
V V mV
4.5V < VCC < 16V, Gate switching 3V < VC < 16V, Gate non-switching VCC = 4,
4.5 2.7 500
6.0 4.0 900
mA mA A
Note 1: Guaranteed by design not 100% tested in production.
Package Pin Description
PACKAGE PIN # PIN SYMBOL FUNCTION
8L SO Narrow & PDIP 1 2 3 4 5 6 7 8 VGATE PGnd COSC Gnd VFB VCC CS VC Driver pin to gate of external PFET. Output power stage ground connection. Oscillator frequency programming capacitor. Logic ground. Feedback voltage input. Logic supply voltage. Soft start and fault timing capacitor. Driver supply voltage.
3
CS51031
Block Diagram
VREF IC COSC 7IC RG Oscillator Comparator A1 G1 VGATE Flip-Flop R F2 G2 1.5V VCC VCC VCCOK VREF 3.3V VREF = 3.3V G3 IT CS CS Comparator A2 IT 5 1.5V 2.5V
Fault Comp
VC
VGATE
Q PGnd Q A6 +
Hold Off Comp
S
VFB Comparator VFB 1.25V
2.5V
+ -
0.7V
G4
+ 1.15V CS Charge Sense Comparator A4 + 2.3V
R F1 G5 S
Q
IT 55
Q
Slow Discharge Flip-Flop A3 + Slow Discharge Comparator Gnd
2.4V
Figure 1: Block Diagram for CS51031
Circuit Description Theory of Operation Control Scheme The CS51031 monitors and the output voltage to determine when to turn on the PFET. If VFB falls below the internal reference voltage of 1.25V during the oscillatorOs charge cycle, the PFET is turned on and remains on for the duration of the charge time. The PFET gets turned off and remains off during the oscillatorOs discharge time with the maximum duty cycle to 80%. It requires 7mV typical, and 20mV maximum ripple on the VFB pin is required to operate. This method of control does not require any loop stability compensation. Startup The CS51031 has an externally programmable soft start feature that allows the output voltage to come up slowly, preventing voltage overshoot on the output. At startup, the voltage on all pins is zero. As VCC rises, the VC voltage along with the internal resistor RG keeps the PFET off. As VCC and VC continue to rise, the oscillator capacitor (COSC ) and the Soft start/Fault Timing capacitor (CS) charges via internal current sources. COSC gets charged by the current source IC and CS gets charged by the IT source combination described by: ICS = IT -
(
IT IT + . 55 5
)
The internal Holdoff Comparator ensures that the external PFET is off until VCS > 0.7V, preventing the GATE flip-flop (F2) from being set. This allows the oscillator to reach its operating frequency before enabling the drive output. Soft start is obtained by clamping the VFB comparatorOs (A6) reference input to approximately 1/2 of the voltage at the CS pin during startup, permitting the control loop and the output voltage to slowly increase. Once the CS pin charges above the Holdoff Comparator trip point of 0.7V, the low feedback to the VFB Comparator sets the GATE flip-flop during COSC Os charge cycle. Once the GATE flip-flop is set, VGATE goes low and turns on the PFET. When VCS exceeds 4
CS51031
Circuit Description: continued 2.4V, the CS charge sense comparator (A4) sets the VFB comparator reference to 1.25V completing the startup cycle. Buck Regulator Operation
Q1 VIN CIN R1 CO R2 RLOAD L
Lossless Short Circuit Protection The CS51031 has OlosslessO short circuit protection since there is no current sense resistor required. When the voltage at the CS pin (the fault timing capacitor voltage ) reaches 2.5V during startup, the fault timing circuitry is enabled. During normal operation the CS voltage is 2.6V. During a short circuit or a transient condition, the output voltage moves lower and the voltage at VFB drops. If VFB drops below 1.15V, the output of the fault comparator goes high and the CS51031 goes into a fast discharge mode. The fault timing capacitor, CS, discharges to 2.4V. If the VFB voltage is still below 1.15V when the CS pin reaches 2.4V, a valid fault condition has been detected. The slow discharge comparator output goes high and enables gate G5 which sets the slow discharge flip flop. The Vgate flip flop resets and the output switch is turned off. The fault timing capacitor is slowly discharged to 1.5V. The CS51031 then enters a normal startup routine. If the fault is still present when the fault timing capacitor voltage reaches 2.5V, the fast and slow discharge cycles repeat as shown in figure 2. If the VFB voltage is above 1.15V when CS reaches 2.4V a fault condition is not detected, normal operation resumes and CS charges back to 2.6V. This reduces the chance of erroneously detecting a load transient as a fault condition.
2.6V
D1
Control
Feedback
Figure 3. Buck regulator block diagram.
A block diagram of a typical buck regulator is shown in Figure 3. If we assume that the output transistor is initially off, and the system is in discontinuous operation, the inductor current IL is zero and the output voltage is at its nominal value. The current drawn by the load is supplied by the output capacitor CO . When the voltage across CO drops below the threshold established by the feedback resistors R1 and R2 and the reference voltage VREF, the power transistor Q1 switches on and current flows through the inductor to the output. The inductor current rises at a rate determined by (VIN-VOUT)/L. The duty cycle (or OonO time) for the CS51031 is limited to 80%. If output voltage remains higher than nominal during the entire COSC change time, the Q1 does not turn on, skipping the pulse.
VCS
2.4V
S1
S2
S1
S2 S3 S3 S1
S2 S3 S3
2.5V
1.5V 0V
TSTART START NORMAL OPERATION td1 tFAULT tRESTART td2 FAULT tFAULT
0V
VGATE
1.25V 1.15V
VFB
Figure 2. Voltage on start capacitor (VGS ), the gate (VGATE ), and in the feedback loop (VFB), during startup, normal and fault conditions.
Applications Information CS51031 Design Example Specifications 12V to 5V, 3A Buck converter VIN = 12V 20% (i.e. 14.4V max., 12Vnom., 9.6V min.) VOUT = 5V 2% IOUT = 0.3A to 3A Output ripple voltage < 50mV max. Efficiency > 80% fSW = 200kHz 5 1) Duty cycle estimates Since the maximum duty cycle D, of the CS51031 is limited to 80% min., it is necessary to estimate the duty cycle for the various input conditions over the complete operating range. The duty cycle for a buck regulator operating in a continuous conduction mode is given by: D= VOUT + VF VIN - VSAT
CS51031
Applications Information: continued where VSAT = Rds(on) IOUT max. and Rds(on)is the value at TJ 100uC. If VF = 0.60V and VSAT = 0.60V then the above equation becomes: DMAX = 5.6 9 5.6 13.8 = 0.62
5) Output capacitor The output capacitor and the inductor form a low pass filter. The output capacitor should have a low ESL and ESR. Low impedance aluminum electrolytic, tantalum or organic semiconductor capacitors are a good choice for an output capacitor. Low impedance aluminum are less expensive. Solid tantalum chip capacitors are available from a number of suppliers and are the best choice for surface mount applications. The output capacitor limits the output ripple voltage. The CS51031 needs a maximum of 20mV of output ripple for the feedback comparator to change state. If we assume that all the inductor ripple current flows through the output capacitor and that it is an ideal capacitor (i.e. zero ESR), the minimum capacitance needed to limit the output ripple to 50mV peak to peak is given by: C= AEI 0.6A = = 7.5F 3Hz) (50 10-3V) 8 fSW AEV 8 (200 10
DMIN =
= 0.40
2) Switching frequency and on and off time calculations Given that fSW = 200kHz and DMAX = 0.80 T= 1 fSW = 5s
TON(max) = T DMAX = 5s 0.62 @ 3s TON(min) = T DMIN = 5s 0.40 = 2s TOFF(max) = TON(min) = 5s - 2s = 3s
The minimum ESR needed to limit the output voltage ripple to 50mV peak to peak is: ESR = AEV 50 10-3 = = 83m1/2 AEI 0.6A
3) Oscillator Capacitor Selection The switching frequency is set by COSC, whose value is given by: 95 10-6 COSC in pF = Fsw 30 10 3 Fsw
(
1+
Fsw 3 10
6
(
)
2
)
The output capacitor should be chosen so that its ESR is less than 83m1/2. During the minimum off time, the ripple current is 0.4A and the output voltage ripple will be: AEV=ESR AEI = 83m1/2 0.4 = 33mV.
4) Inductor selection The inductor value is chosen for continuous mode operation down to 0.3Amps. The ripple current AEI = 2 IOUTmin = 2 0.3A = 0.6A. Lmin = (VOUT + VD) TOFF(max) = AEI 5.6V 3s 0.6A =28H
6) VFB divider VOUT = 1.25V
(
R1 + R2 R1 = 1.25V +1 R2 R2
)
(
)
This is the minimum value of inductor to keep the ripple current to <0.6A during normal operation. A smaller inductor will result in larger ripple current. Ripple current at a minimum off time is AEI = (VOUT + VF) TOFF(min) = LMIN 5.6V 2s 28H =0.4A
The input bias current to the comparator is 4A. The resistor divider current should be considerably higher than this to ensure that there is sufficient bias current. If we choose the divider current to be at least 250 times the bias current this permits a divider current of 1mA and simplifies the calculations. 5V = R1 + R2 = 5k1/2 1mA Let R2 = 1K Rearranging the divider equation gives: R1 = R2
The core must not saturate with the maximum expected current, here given by: IMAX = IOUT + AEI/2 = 3A+0.4A/2 = 3.2A
(
VOUT 5V - 1 = 1k1/2 - 1 = 3k1/2 1.25 1.25
)
(
)
6
CS51031
Applications Information: continued 7) Divider bypass Capacitor Crr Since the feedback resistors divide the output voltage by a factor of 4, i.e. 5V/1.25V= 4, it follows that the output ripple is also divided by four. This would require that the output ripple be at least 60mV (4 15mV) to trip the feedback comparator. We use a capacitor Crr to act as an AC short . The ripple voltage frequency is equal to the switching frequency so we choose Crr = 1nF. 8) Soft start and Fault timing capacitor CS. CS performs several important functions. First it provides a delay time for load transients so that the IC does not enter a fault mode every time the load changes abruptly. Secondly it disables the fault circuitry during startup, it also provides soft start by clamping the reference voltage during startup, allowing it to rise slowly, and, finally it controls the hiccup short circuit protection circuitry. This reduces the duty cycle to approximately 0.035 during short circuit conditions. An important consideration in calculating CS is that itOs voltage does not reach 2.5V (the voltage at which the fault detect circuitry is enabled) before VFB reaches 1.15V otherwise the power supply will never start. If the VFB pin reaches 1.15V, the fault timing comparator will discharge CS and the supply will not start. For the VFB voltage to reach 1.15V the output voltage must be at least 4 1.15 = 4.6V. If we choose an arbitrary startup time of 900s, the value of CS is: t Startup= CS 2.5V ICharge tCharge(t) = CS (2.5V - 1.5V) ICharge
Where ICharge is 264A typical. tCharge(t) = CS 3787 The fault time is given by: tFault = CS (3787 + 1515 + 1.5 105) tFault = CS (1.55 105) For this circuit tFault = 0.1 10-6 1.55 105 = 15.5S A larger value of CS will increase the fault time out time but will also increase the soft start time. 9) Input Capacitor The input capacitor reduces the peak currents drawn from the input supply and reduces the noise and ripple voltage on the VCC and VC pins. This capacitor must also ensure that the VCC remains above the UVLO voltage in the event of an output short circuit. A low ESR capacitor of at least 100F is good. A ceramic surface mount capacitor should also be connected between VCC and ground to filter high frequency noise. 10) MOSFET Selection The CS51031 drives a P-channel MOSFET. The VGATE pin swings from Gnd to VC. The type of PFET used depends on the operating conditions but for input voltages below 7V a logic level FET should be used. A PFET with a continuous drain current (ID) rating greater than the maximum output current is required. The Gate-to-Source voltage VGS and the Drain-to Source Breakdown Voltage should be chosen based on the input supply voltage. The power dissipation due to the conduction losses is given by: PD = IOUT2 RDS(ON) D where RDS(ON) is the value at TJ = 100uC.
CSmin =
900s 264A = 950nF @ 0.1F 2.5V
The fault time is the sum of the slow discharge time the fast discharge time and the recharge time. It is dominated by the slow discharge time. The first parameter is the slow discharge time, it is the time for the CS capacitor to discharge from 2.4V to 1.5V and is given by: CS (2.4V - 1.5V) tSlowDischarge(t) = IDischarge Where IDischarge is 6A typical. tSlowDischarge(t) = CS 1.5 105 The fast discharge time occurs when a fault is first detected. The CS capacitor is discharged from 2.5V to 2.4V. CS (2.5V - 2.4V) IFastDischarge
The power dissipation of the PFET due to the switching losses is given by: PD = 0.5 VIN IOUT (tr ) fSW Where tr = Rise Time. 11) Diode Selection The flyback or catch diode should be a Schottky diode because of itOs fast switching ability and low forward voltage drop. The current rating must be at least equal to the maximum output current. The breakdown voltage should be at least 20V for this 12V application. The diode power dissipation is given by: PD = IOUT VD (1 - Dmin)
tFastDischarge(t) =
Where I FastDischarge is 66A typical. tFastDischarge(t) = CS 1515 The recharge time is the time for CS to charge from 1.5V to 2.5V. 7
CS51031
Package Specification
PACKAGE DIMENSIONS IN mm (INCHES) PACKAGE THERMAL DATA
D Lead Count 8L SO Narrow 8L PDIP Metric Max Min 5.00 4.80 10.16 9.02 English Max Min .197 .189 .400 .355
Thermal Data RQJC typ RQJA typ
8L SO Narrow 45 165
8L PDIP 52 100
uC/W uC/W
8L SO Narrow; 150 mil wide
4.00 (.157) 3.80 (.150)
6.20 (.244) 5.80 (.228)
0.51 (.020) 0.33 (.013)
1.27 (.050) BSC
1.75 (.069) MAX 1.57 (.062) 1.37 (.054) 1.27 (.050) 0.40 (.016) 0.25 (.010) 0.19 (.008) D REF: JEDEC MS-012
0.25 (0.10) 0.10 (.004)
8L PDIP; 300 mil wide
7.11 (.280) 6.10 (.240)
8.26 (.325) 7.62 (.300) 3.68 (.145) 2.92 (.115)
1.77 (.070) 1.14 (.045)
2.54 (.100) BSC
.356 (.014) .203 (.008)
0.39 (.015) MIN. .558 (.022) .356 (.014) Some 8 and 16 lead packages may have 1/2 lead at the end of the package. All specs are the same.
REF: JEDEC MS-001
D
Ordering Information
Part Number CS51031YD8 CS51031YDR8 CS51031YN8
Rev. 2/13/98
Description 8L SO Narrow 8L SO Narrow (tape & reel) 8L PDIP 8
Cherry Semiconductor Corporation reserves the right to make changes to the specifications without notice. Please contact Cherry Semiconductor Corporation for the latest available information.
(c) 1999 Cherry Semiconductor Corporation

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