SP6127 datasheet(6/12 Pages) EXAR

6 / 12 page

Jan28-08 RevE

SP6127 TSOT-6 PFET Buck Controller

 2007 Exar Corporation

Page 6

Overcurrent Protection (OCP)

SP6127

Gate

30uA

Vin - 0.3V

Vin

Ov er-Current Comparator

Figure 3 Overcurrent protection circuit

The

overcurrent

protection

(OCP)

circuit

functions by monitoring the voltage across the

high-side FET Q1. When this voltage exceeds

0.3V, the overcurrent comparator triggers and

the controller enters hiccup mode. For example

if Q1 has RDS(ON)=0.1 , then the overcurrent

will trigger at I = 0.3V/0.1 =3A. To program a

lower overcurrent use a resistor Rs as shown in

figure 1. Calculate Rs from:

(

)

Rds

Iout

)

(

−

...(5)

Where:

1.15 is used to calculate peak inductor current

which is nominally 15% higher than average

output current

RDS(ON) is MOSFET ON-resistance rating

Kt is a multiplier that accounts for increase in

RDS(ON) due to temperature

Example: A switching

MOSFET

used with

SP6127 has RDS(ON) of 0.08

and Kt is 1.5.

Program

the

over-current

circuit

that

maximum output is 2A.

(

)

Ohm

−

Rs=800

Using

the

above

equation

there

good

agreement between calculated and test results

for Rs in the range of 0.5k

to 3k . For Rs larger

than 3k , test results are lower than those

predicted

(5),

due

circuit

parasitics.

Therefore the maximum value of Rs should be

limited to 3k .

Note that in order to safeguard against false

overcurrent trigger due to transients, there is a

150ns delay between the turn on of the MOSFET

and

when

OCP

circuit

activated.

consequence at very high Vo/VIN ratio, where

MOSFET on-time is less than 150ns, the OCP

circuit will not detect overcurrent.

Using the ON/OFF Function

The Feedback pin serves a dual role of ON/OFF

control. The MOSFET driver is disabled when a

voltage greater than 1V is applied at the FB pin.

Maximum voltage rating of this pin is 5.5V. The

controlling signal should be applied through a

small signal diode as shown on page 1.

Please note that an optional 10k

bleeding

resistor across the output helps keep the output

capacitor discharged under no load condition.

Programming the Output Voltage

To program the output voltage, calculate R2

using the following equation:













−

Vref

Vout

Where:

VREF=0.6 is the reference voltage of the SP6127

R1=200k

is a fixed-value resistor that, in

addition to being a voltage divider, it is part of the

compensation

network.

order

simplify

compensation calculations, R1 is fixed at 200k .

Soft Start

Soft Start is preset internally to 5ms (nominal).

Internal Soft Start eliminates the need for the

external capacitor CSS that is commonly used to

program this function.

______________________________________________________ GENERAL OVERVIEW

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