MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER
after a delay. The PG delay time is about 1/4 of
Over-Current Protection
SS time. If VFB falls below 80% of the reference
voltage, PG pin is pulled down to ground by an
internal MOSFET.
The MPQ28261 has hiccup over-current limit
protection for when the inductor current peak
value exceeds the set current limit threshold:
hiccup occurs when the output voltage drops
below 70% of the reference voltage and the
inductor current exceeds the current limit. This
is especially useful to ensure system safety
under fault conditions. The hiccup function is
enabled during SS duration.
External Soft-Start
Connecting a capacitor from the soft-start (SS)
pin to ground adjusts the SS time. During SS
initiation, an internal 12µA current source
charges the external capacitor. The SS pin
connects internally to the non-inverting input of
the error amplifier. The SS continues to charge
until the voltage on the SS capacitor exceeds
the 0.6V reference voltage. Then the reference
voltage functions as the non-inverting input of
the error amplifier. The following equation
estimates the SS time:
Current Limit
IINDUCTOR
0A
0.6V × CSS (nF)
tSS (ms) =
12μA
VOUT
Table 1 lists SS times with different external
capacitor values. If the output capacitor is large,
use a large SS capacitor to ensure proper
device start-up.
0V
t
Figure 3: Over-Current Behavior in Hiccup
Figure 3 shows the hiccup function. After the
hiccup occurs, the SS pin voltage goes low and
MPQ28261 begins SS shortly after. During the
SS period, if the inductor current hits current
limit and the output voltage is less than 70% of
the reference, the SS voltage will go low.
Table 1: Soft Start Time vs. Capacitor Value
tSS (ms)
0.5
CSS (nF)
10
1.65
2.35
3.4
33
47
68
Thermal Shutdown
5
100
Thermal shutdown prevents the chip from
operating at exceedingly high temperatures.
If the output of the MPQ28261 is pre-biased to
a certain voltage during startup, the IC will
disable switching on both the HS-FET and LS-
FET until the voltage on the internal SS
capacitor exceeds the sensed output VFB, as
Figure 2 shows.
EN
FB
Vref takes
control after SS
Pre-bias
SS
SW
Figure 2: Pre-Biased Soft-Start
MPQ28261 Rev. 1.12
10/10/2013
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