SP6121 Datasheet, PDF (4/16 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller Ideal for 2A to 10A, Small Footprint, DC-DC Power Converters

FUNCTIONAL DIAGRAM
1.25V
Reference
FAULT
0.4V/ms
SOFTSTART
V_FB 3
COMP 4
GM
ERROR
+ ^AMP
-
1V _-
+
DRIVER ENABLE
-
300mV +
5µA _{PWM COMP}
-
+
SHUTDOWN
RESET
Dominant
S
Q
PWM
Logic
R
V_CC 1
ISET 6
30µA
(3300 ppm/°C)
ISENSE 5
UVLO
-
750mV RAMP
F = 500kHz
2.79V ON
2.69V OFF
+
Reset
Dominant
PDRV
Over Current
(Gated S&H)
+
X 3.3
+
-
530mV
-
(3300 ppm/°C)
SHUTDOWN
S
FAULT
Q
R
V_CC
Synchronous
Driver
8 PDRV
7 NDRV
2 GND
COMP
THEORY OF OPERATION
General Overview
The SP6121 is a constant frequency, voltage
mode, synchronous PWM controller designed
for low voltage, DC/DC step down converters.
It is intended to provide complete control for a
high power, high efficiency, precisely regulated
output voltage from a highly integrated 8-pin
solution.
The internal free-running oscillator accurately
sets the PWM frequency at 500kHz without
requiring any external elements and allows the
use of physically small, low value external com-
ponents without compromising performance. A
transconductance amplifier is used for the error
amplifier, which compares an attenuated sample
of the output voltage with a precision reference
voltage. The output of the error amplifier
(COMP), is compared to a 0.75V peak-to-peak
ramp waveform to provide PWM control. The
COMP pin provides access to the output of the
error amplifier and allows the use of external
components to stabilize the voltage loop.
High efficiency is obtained through the use of
synchronous rectification. Synchronous regula-
tors replace the catch diode in the standard buck
converter with a low R_DS(ON) N-channel
MOSFET switch allowing for significant effi-
ciency improvements. The SP6121 includes two
fast MOSFET drivers with internal non-overlap
circuitry and drives a complementary pair of
power transistors, P-channel on the high side,
and N-channel on the low side. The use of a P-
channel high side device minimizes complexity
and external component count by eliminating
the need for a charge pump that would otherwise
be required to fully enhance an N-channel de-
vice. It also allows inherent 100% duty cycle for
low dropout operation in the event of a low input
supply voltage condition.
The SP6121 includes an internal 0.4V/ms soft-
start circuit that provides controlled ramp up of
the output voltage, preventing overshoot and
inrush current at power up.
Current limiting is implemented by monitoring
the voltage drop across the R_DS(ON) of the high
side P-channel MOSFET while it is conducting,
thereby eliminating the need for an external
sense resistor. The over-current comparator has
a built-in threshold of 160mV that can be pro-
grammed to higher values using a single exter-
nal resistor, connected to the I_SET pin, whose
Date: 5/25/04
SP6121 Low Voltage, Synchronous Step Down PWM Controller
4
© Copyright 2004 Sipex Corporation