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

MPQ4480 Rev. 1.0

www.MonolithicPower.com

16

7/2/2019

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falls below 105% of VREF. During OV protection,

if the input voltage reaches 40V, the MPQ4480

stops the OVP discharge function until VIN

drops to 38V.

RGND Short to Battery Protection

The MPQ4480 supports USB_GND short to

battery protection. The USB_GND should be

connected to the RGND through an N-MOSFET.

The MPQ4480 senses the current from RGND

to PGND, and the GATE pin is pulled low to

turn off the N-MOSFET when the RGND-to-

PGND current is above 12A. If RGND shorts to

battery, the buck stops switching. When the

battery short to ground is removed, the

MPQ4480 resumes normal operation.

Output Line Drop Compensation

The MPQ4480 is capable of compensating an

output voltage drop, such as high impedance

caused by a long trace, to keep a fairly constant

load-side voltage.

The MPQ4480 uses the sensed load current

through the internal ground current sensing

MOSFET to sink a current (IFB) at the FB pin

(see Figure 5).

-

+

VREF

R1

V+

GND

R2

CO

L

Gate

Drive

Error

Amp

SW

RGND

FB

VIN

PGND

IRTN

IFB

G

VDROP

RSENSE

Figure 5: Output Line Drop Compensation Circuit

Calculate the feedback current with Equation

(2):



FB

RTN

IG I

(2)

VO can be calculated with Equation (3):













1

OUT

REF

1

RTN

2

R

V1 v

R G I

R

(3)

The line drop compensation amplitude at

certain output current condition is equal to R1 *

G * IOUT. The VDROP pin can program three

gain levels, as the specification table shows.

The R1 value can also be used to adjust the

line drop compensation amplitude. For example,

at 3A output current, pull VDROP = 0 and select

40.2kΩ for R1 for the line drop compensation to

be ~73mV (typically).

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. The

UVLO’s rising threshold is 2.2V, with a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN and VCC

through D1, D2, M1, C4, L1, and C2 (see

Figure 6). The BST capacitor (C4) voltage is

charged quickly by turning on M1 when the low-

side switch is turned on. The 2.5μA input to the

BST current source also charges the BST

capacitor when the low-side switch does not

turn on.

VIN

VCC

SW

VOUT

Turn on PMOS

when LS is on

L1

C2

M1

D1

2.5µA

D2

C4

BST

Figure 6: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

If both VIN and EN exceed their respective

thresholds, the chip is enabled. The reference

block starts first, generating a stable reference

voltage and currents, and then the internal

regulator is enabled. The regulator provides a

stable supply for the remaining circuitries.

Several events can shut down the chip: EN low,

VIN low, and thermal shutdown. In shutdown,

the signaling path is blocked to avoid any fault

triggering. Then the COMP voltage and the

internal supply rail are pulled down. The floating

driver is not subject to this shutdown command.

Thermal Shutdown (TSD)

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.