Date: 5/25/04

SP6121 Low Voltage, Synchronous Step Down PWM Controller

© Copyright 2004 Sipex Corporation

13

external voltage reference programs the output voltages less than 1.25V.

®

®

®

®

AB

SP6121

SP6121

R1

R2

R1

R2

R3

PDRV

SP6121

NDRV

L1

RSET

Q1

Q2

+

-

+

-

160mV

30µA

Figure 6. Current Limit Setting

30

according to the following expression:

where,

Kelvin-Sense connections should be made di-

rectly at the drain and source of Q1.

SP6121 provides two additional features that

enhance the performance of the overcurrent

function. First, an internal sample and hold filter

connected after the main current-sense com-

parator, prevents that noise spikes or very short

and mild overload conditions, that could occur

during a load transient, spuriously activate the

current limit circuitry. This typically eliminates

the need of using any external filtering that would

be otherwise required. Additionally, since the

both the 30

and the 160mV built-in current limit threshold

have been designed with a positive temperature

coefficient of about 0.33%/C to provide first order

correction for current limit versus temperature.

This compensation relies on the high amount of

thermal coupling that typically exists between the

high side switch Q1 and the SP6121 due to the

compact size of the power supply. With this first

order compensation, the current limit trip point

does not need to be set to an increased level at room

temperature to guarantee a desired output current

level at higher temperatures.

Output Voltage Program

As shown in Figure 7(A), the voltage divider

voltage according to

R2

where 1.25V is the internal reference voltage.

Select R2 in the range of 10k to 100k, and R1 can

be calculated using

R1 =

1.25

For output voltage less than 1.25V, a simple

circuit shown in Figure 7(B) can be used in

greater than 1.25V. For simplicity, use the same

resistor value for R1 and R2, then R3 is deter-

mined as follows,

R3 =

APPLICATIONS INFORMATION: Continued