CS51031YD8 datasheet(7/8 Pages) CHERRY

Nome de Peças	CS51031YD8

Descrição Electrónicos	Fast PFET Buck Controller Does Not Require Compensation

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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 com-

parator. We use a capacitor Crr to act as an AC short .

The ripple voltage frequency is equal to the switching fre-

quency 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 itÕs

voltage does not reach 2.5V (the voltage at which the fault

detect circuitry is enabled) before VFB reaches 1.15V other-

wise 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 900µs, the value of

CS is:

t Startup=

CSmin =

= 950nF

@ 0.1µF

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:

tSlowDischarge(t) =

Where IDischarge is 6µA typical.

tSlowDischarge(t) = CS ´ 1.5 ´ 105

The fast discharge time occurs when a fault is first detect-

ed. The CS capacitor is discharged from 2.5V to 2.4V.

tFastDischarge(t) =

Where I FastDischarge is 66µA typical.

tFastDischarge(t) = CS ´ 1515

The recharge time is the time for CS to charge from 1.5V to

2.5V.

tCharge(t) =

Where ICharge is 264µA 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 = 15.5µS

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

100µF 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 = 100ûC.

The power dissipation of the PFET due to the switching loss-

es 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 itÕs fast switching ability and low forward volt-

age 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)

CS ´ (2.5V - 1.5V)

ICharge

CS ´ (2.5V - 2.4V)

IFastDischarge

CS ´ (2.4V - 1.5V)

IDischarge

900µs

´ 264µA

2.5V

CS ´ 2.5V

ICharge

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