APPLICATIONS INFORMATION
Overcurrent Protection
Output Voltage Program
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
As shown in Figure 5, the voltage divider con-
necting to the VFB pin programs the output
voltage according to
R1
R2
VOUT = 0.8(1 +
)
where 0.8V is the internal reference voltage.
SelectR2intherangeof10kto100k, andR1can
be calculated using
R2(VOUT – 0.8)
R1 =
0.8
VOUT
V
CC and SWN pins.
The overcurrent threshold can be calculated as
®
®
R1
R2
VFB
200mV
RDS(ON)
SP6123
IMAX
=
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.
Figure 5. A voltage divider connected to the VFB pin
programs the output voltage.
Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
© Copyright 2004 Sipex Corporation
15
SIPEX [ SIPEX CORPORATION ]
