OPERATION
clamp and then gradually charges preventing
the error amplifier from forcing the loop to
maximum duty cycle. As the COMP voltage
crosses about 1V (valley voltage of the PWM
ramp), the driver begins to switch the high side
MOSFET with narrow pulses in an effort to
keep the converter output regulated . The SP6123
operates at low duty cycle as the COMP voltage
increases above 1V. As the error amp reference
ramps upward, the driver pulses widen until a
steady state value is reached and the output
voltage is regulated to the final value ending the
soft start charge cycle.
COMP
1V
0.7 V
0.3 V
0V
Internal SS
Voltage
Hiccup Mode
When the converter enters a fault mode, the
SP6123 holds the high side and low side
MOSFETs off for a finite period of time. Pro-
vided that the SP6123 is enabled, this time is set
by the internal charge of the soft-start capacitor.
In the event of an overcurrent condition, the
current sense comparator sets the fault latch,
which in turn discharge the internal SS capaci-
tor, the COMP pin and holds the output drivers
off. During this condition, the SP6123 stays off
for the time it takes to discharge the COMP pin
down to the 0.27V shutdown threshold. At this
point, the fault latch is reset, but before the
SP6123 is allowed to attempt restart, the COMP
pin has to charge back to 1V before any output
switching can be initiated. Then, the regulator
attempts to restart normally by delivering short
gate pulses and if the overcurrent condition is
still present, the cycle will repeat itself. How-
ever, if upon restart, the overcurrent condition is
still present, the SP6123 will detect the fault and
remain in a fault state until COMP reaches about
V
CC
-1V thereby increasing the MOSFET off-
time. This protection scheme minimizes ther-
mal stress to the regulator components as the
overcurrent condition persists.
The simplified waveforms that describe the hic-
cup mode operation are shown below.
V
CC
-V
SWN
200 mV
Error Amp
Reference
Voltage
0.8 V
V
OUT
= V
REF
* (1+R1/R2)
0V
I(L)
Inductor
Current
0V
0A
V(V
CC
)
FAULT
V
COMP
3V
0V
V(V
CC
)
SWN
Voltage
0V
TIME
1.0 V
0.3 V
V
BST
GH
Voltage
V
SWN
TIME
Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
© Copyright 2004 Sipex Corporation
6
SIPEX [ SIPEX CORPORATION ]
