Datasheet

MAX17598/MAX17599
Low I
Q
, Wide-Input Range, Active Clamp
Current-Mode PWM Controllers
19Maxim Integrated
Output Short-Circuit Protection
with Hiccup Mode
When the MAX17598/MAX17599 detect eight consecu-
tive peak-current-limit events, both NDRV and AUXDRV
driver outputs are turned off (hiccup is followed by soft-
stop) for a restart period, t
RSTR
. After t
RSTR
, the device
turns on again with a soft-start. The duration of the restart
period is 32678 clock cycles, and therefore depends on
the switching frequency setting. The device also features
a runaway current limit setting at 120% (typ) of the peak
current limit. This feature is useful under short-circuit
faults in forward converters with synchronous rectifiers
that occur during minimum on-time conditions at high
input voltages. Under these conditions, the primary peak
current tends to build up and staircase beyond the peak
current limit setting due to insufficient discharging of the
output inductor. One single event of a runaway current
limit forces the MAX17598/MAX17599 into hiccup mode
operation. Figure 8 shows the behavior of the device prior
and during hiccup mode.
Oscillator Synchronization
The internal oscillator can be synchronized to an external
clock by applying the clock to SYNC/DITHER directly.
The external clock frequency can be set anywhere
between 1.1x to 1.3x the internal clock frequency. Using
an external clock increases the maximum duty cycle by
a factor equal to f
SYNC
/f
SW
.
Frequency Dithering for Spread-Spectrum
Applications (Low EMI)
The switching frequency of the converter can be dith-
ered in a range of Q10% by connecting a capacitor from
DITHER/SYNC to GND, and a resistor from DITHER to RT
as shown in the Typical Applications Circuit. This results
in lower EMI.
Figure 8. Hiccup-Mode Timing Diagram
CURRENT-SENSE
VOLTAGE
HICCUP SIGNAL
SOFT-START
VOLTAGE, V
SS
V
SS-HI
HICCUP TIMEOUT
t
SS
t
RSTR
DISCHARGE WITH
ISSDISCH
V
CS-PEAK
(305mV)