MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MPQ28261 Rev. 1.12

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13

10/10/2013

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after a delay. The PG delay time is about 1/4 of

voltage, PG pin is pulled down to ground by an

internal MOSFET.

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:

SS

SS

0.6V C (nF)

t(ms)

12 A

×

=

μ

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.

Table 1: Soft Start Time vs. Capacitor Value

0.5

10

1.65

33

2.35

47

3.4

68

5

100

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

Figure 2 shows.

Pre-bias

EN

SS

SW

FB

Vref takes

control after SS

Figure 2: Pre-Biased Soft-Start

Over-Current Protection

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.

0A

0V

Current Limit

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.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.