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CS51033YDR8

Fast PFET Buck Controller Does Not Require Compensation

Cherry semiconductor 

Applications Information: continued

to rise slowly and finally it controls the Hiccup short circuit

protection circuitry. This function reduces the PFET's duty

cycle to 2% of the CS period.

The most important consideration in calculating CS is that

it’s 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.

For this circuit

TFAULT = 0.1 × 10-6 × 1.55 × 105 = 0.0155

A larger value of CS will increase the fault time out time but

will also increase the soft start time.

8) Input Capacitor.

If the VFB pin reaches 1.15V the fault timing comparator will

discharge CS and the supply will not start. For the VFB volt-

age to reach 1.15V the output voltage must be at least 4 ×

1.15 = 4.6V.

If we choose an arbitrary startup time of 200µs we calculate

the value of CS from:

CS × 2.5V

T = ICHARGE

Use 0.1µf.

200µs × 264µA

CS(min) =

2.5V

= 0.02µF

The fault time out time is the sum of the slow discharge

time the fast discharge time and the recharge time and is

obviously 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 = ΙDISCHARGE

Where IDISCHARGE is 6µA typical.

TSLOWDISCHARGE = CS × 1.5V × 105

The fast discharge time occurs when a fault is first detected.

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

TFASTDISCHARGE =

CS × (2.5V - 2.4V)

ΙFASTDISCHARGE

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. CIN should be a low ESR capacitor

of at least 100µf. A ceramic surface mount capacitor should

also be connected between VCC and ground to prevent

spikes.

9) MOSFET Selection

The CS51033 drive 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.

Choose a PFET with a continuous drain current (Id) rating

greater than the maximum output current. RDS(on) should be

less than

RDS < =

0.6V

IOUT(max)

167mΩ

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

=

2

OUT

×

RDS(on)

×

D

The power dissipation due to the switching losses is given

by:

Where IFASTDISCHARGE is 66µA typical.

TFASTDISCHARGE = CS × 1515

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

2.5V.

TCHARGE

=

CS

× (2.5V-1.5V)

ΙCHARGE

Where ICHARGE is 264µA typical.

TCHARGE = CS × 3787

The fault time out time is given by:

TFAULT = CS × (3787 + 1515 + 1.5 × 105)

TFAULT = CS × 1.55 × 105

PD = 0.5 × VIN × IOUT × (TRr + TF) × FSW

Where tr =Rise Time and tf= Fall Time.

10) Diode Selection.

The flyback or catch diode should be a Schottky diode

because of it’s 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)

7

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