SP6125 datasheet(6/12 Pages) SIPEX

6 / 12 page

Mar22-07 RevC

SP6125: TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

Overcurrent protection

Figure 3- Overcurrent protection circuit

The overcurrent protection 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)

The overcurrent circuit triggers at peak current

through Q1 which is usually about 15% higher

than

average

output

current.

Hence

the

multiplier 1.15 is used in (5).

Example: A switching MOSFET used with

SP6125 has Rds(on) of 0.1 . Program the over-

current circuit so that maximum output is 2A.

(

)

Ohm

−

Rs = 2333

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 by (5), due to circuit parasitics.

Therefore maximum value of Rs should be

limited to 3k .

Using the ON/OFF Function

The Feedback pin serves a dual role of

ON/OFF

control.

The

MOSFET

driver

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 SP6125

R1=200k

is a fixed-value resistor that, in

addition to being a voltage divider, it is part of

the compensation network. In order to 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.

MOSFET Gate Drive

P-channel drive is derived through an internal

regulator that generates VIN-5V. This pin (VDR)

has to be connected to VIN with a 0.1uF

decoupling capacitor. The gate drive circuit

swings between VIN and VIN-5 and employs

powerful drivers for efficient switching of the P-

channel MOSFET.

SP6125

Gate

30uA

Vin - 0.3V

Vin

Ov er-Current Comparator

General Overview

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