SP6123 Datasheet, PDF (15/18 Pages) Sipex Corporation – Low Voltage, Synchronous Step-Down PWM Controller Ideal for 2A to 10A, Small Footprint, DC-DC Power Converters

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 R_DS(ON) detection. By using the R_DS(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. R_DS(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
R_DS(ON) of MOSFET varies. The SP6123 pro-
vides a built-in 200mV threshold between the
V_CC and SWN pins.
The overcurrent threshold can be calculated as
I_MAX =
200mV
R_DS(ON)
To ensure accurate current sensing, the V_CC pin
should be connected directly to the drain of the
high side MOSFET. A RC filter on the V_CC 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.
APPLICATIONS INFORMATION
Output Voltage Program
As shown in Figure 5, the voltage divider con-
necting to the V_FB pin programs the output
voltage according to
V_OUT = 0.8(1 + ^RR12)
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
V_OUT
®
R1
SP6123
V_FB
R2
Figure 5. A voltage divider connected to the V_FB pin
programs the output voltage.
Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
15
© Copyright 2004 Sipex Corporation