MPQ4480 – 36V, 6A, SYNC STEP-DOWN CONVERTER FOR AEC1-Q100 QUALIFIED

MPQ4480 Rev. 1.0

www.MonolithicPower.com

15

7/2/2019

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Frequency Spread Spectrum

The MPQ4480-FD is the switching frequency

spread spectrum version of the MPQ4480. The

purpose of spread spectrum is to minimize the

peak emissions at a specific frequency.

The MPQ4480-FD uses a 4kHz triangle wave

(rising 125µs, falling 125µs) to modulate the

internal oscillator (see Figure 4). The frequency

span of spread spectrum operation is ±10%.

t

250μs

fSW =

450kHz

fSW = 495kHz

fSW =

405kHz

Fundamental

Frequency

Figure 4: Frequency Spread Spectrum

The MPQ4480-FD frequency is only determined

by internal trimming. It has no relationship with

the FREQ pin’s status.

Frequency Synchronizing

The MPQ4480 can be synchronized to an

external clock with a range from 300kHz to

2.1MHz through the SYNC pin, in series with a

10pF capacitor. The internal clock rising edge is

synchronized to the external clock rising edge.

A typical SYNC input signal should have a

<25% duty cycle.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The UVLO comparator monitors the input

voltage. The UVLO rising threshold is 3.95V; its

falling threshold is 3.45V.

Internal Soft Start (SS)

The soft start (SS) prevents the converter

output voltage from overshooting during start-

up. When the chip starts up, the internal

circuitry generates a SS voltage that ramps up

from 0V to 5.05V. When VSS is lower than VREF,

the error amplifier uses VSS as the reference.

When VSS is higher than VREF, the error

amplifier uses VREF as the reference. The SS

time is internally set to 1.1ms.

If the output of the MPQ4480 is pre-biased to a

certain voltage during start-up, the IC disables

the switching of both the high-side and low-side

switches until the voltage on the internal SS

capacitor exceeds the internal feedback voltage.

EN Shutdown Discharge

When EN is pulled low, the MPQ4480 enters

output discharge mode. Meanwhile, the internal

soft-start capacitor starts to discharge. The part

remains in discharge mode until the soft-start

capacitor discharges to very low.

In this mode, the low-side switch remains on

until the low-side current reaches -2A, then

turns on again after a clock cycle.

CC Mode Over-Current Protection

The MPQ4480 senses the ground current and

uses this information to limit output current

below a certain threshold. If IOUT exceeds the

set current-limit threshold, the MPQ4480 enters

constant current limit mode (CC mode). In this

mode, the current amplitude is limited. As the

load resistance reduces, the output voltage

drops until the feedback voltage falls below the

under-voltage (UV) threshold (typically 30%

below the reference).

Once UV triggers, the MPQ4480 enters hiccup

mode to periodically restart the part. This

protection mode is especially useful when the

output is dead-shorted to ground. This greatly

reduces the average short-circuit current,

alleviates thermal issues, and protects the

regulator. The MPQ4480 exits hiccup mode

once the over-current condition is removed.

In addition to the ground current limit, the

MPQ4480 also has a high-side peak current

limit.

Pulling the ILIM pin to GND, to VCC, or floating

it can set three different CC current-limit

thresholds.

Over-Voltage Protection (OVP)

The MPQ4480 monitors a resistor-divided

feedback voltage to detect over-voltage. When

the feedback voltage (VFB) exceeds 115% of the

target voltage, the controller enters a dynamic

regulation period. During this period, the LS-

FET remains on until its current goes to -2A.

This discharges the output and tries to keep it

within the normal range. If the OV still exists,

the LS-FET turns on again after an 800ns delay.

The part exits this regulation period when VFB