SP6123 datasheet(15/18 Pages) SIPEX

15 / 18 page

Date: 5/25/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

Figure 5. A voltage divider connected to the VFB pin

programs the output voltage.

VFB

VOUT

SP6123

Overcurrent Protection

Over current protection on the SP6123 is imple-

mented through detection of an excess voltage

condition across the high side switch during

conduction. This is typically referred to as high

side RDS(ON) detection. By using the RDS(ON) of

Q1 to measure the output current, the current

limit circuit eliminates the sense resistor that

would otherwise be required and the corre-

sponding loss associated with it. This improves

the overall efficiency and reduces the number of

components in the power path benefiting size

and cost. RDS(ON) sensing is by default inaccu-

rate and is primarily meant to protect the power

supply during a fault condition. The overcurrent

trip point will vary from unit to unit as the

RDS(ON) of MOSFET varies. The SP6123 pro-

vides a built-in 200mV threshold between the

VCC and SWN pins.

The overcurrent threshold can be calculated as

IMAX =

200mV

RDS(ON)

To ensure accurate current sensing, the VCC pin

should be connected directly to the drain of the

high side MOSFET. A RC filter on the VCC pin

is not recommended because it would artifi-

cially alter the current signal and reduce the

overcurrent threshold from the value given by

the equation.

Output Voltage Program

As shown in Figure 5, the voltage divider con-

necting to the VFB pin programs the output

voltage according to

VOUT = 0.8(1 +

)

where 0.8V is the internal reference voltage.

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

be calculated using

R1 = R2(VOUT – 0.8)

0.8

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