MP5403B

– DUAL BUCK AND ONE LOAD SWITCH PMIC

MP5403B Rev. 1.0

www.MonolithicPower.com

17

12/5/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP5403B has two step-down regulators and

one load switch integrated into an ultra-small

UTQFN-20 package. The two buck regulators

are able to run up to a peak 5A and peak 4A load

current, respectively. With a peak-current-mode

control scheme and an interleaving PWM clock,

the MP5403B minimizes the input voltage ripple

and achieves a fast dynamic load response. A

3A load switch with only 20

achieve extremely small conduction loss and

provide tight regulation with a high load current.

The load switch can also clamp VOUT to 5.5V.

The MP5403B can be used in compact solid-

state drives (SSD), portable instruments, and

battery-powered devices.

Peak-Current-Mode Control

The two buck regulators of the MP5403B

operate at an 180° phase shift to reduce the

input current ripple and the required input

capacitor.

In

continuous

conduction

mode

(CCM), two internal clocks control the switching

behavior. The high-side MOSFET (HS-FET)

turns on at the corresponding

clock’s rising edge.

Two clocks are at an 180° phase shift. With the

high-side switch current increases and reaches

the internal compensation voltage, the high-side

switch is turned off, and the low-side switch is

turned on to conduct current.

Figure 2: Phase Shift

The switching frequency is 1.5MHz, typically,

running in CCM. With a lower input voltage, the

switching frequency falls and works with a large

duty cycle and a fixed off-time mode.

Light-Load Operation

In light-load mode, the MP5403B uses a

proprietary control scheme to save power and

improve efficiency. The MP5403B turns off the

low-side switch when the inductor current begins

reversing.

Then

MP5403B

works

in

discontinuous

conduction

mode

(DCM)

operation. With light-load mode control, the

switching loss can be reduced greatly due to the

lower switching frequency.

A zero-current cross detection (ZCD) circuit is

used to detect if the inductor current begins

reversing.

Considering

the

internal

circuit

propagation time, the typical delay is 50ns. This

means that the inductor current continues falling

after ZCD is triggered in this delay. If the inductor

close to VIN), the low-side MOSFET (LS-FET) is

turned off, and the inductor current may be

negative. This prevents the MP5403B from

entering DCM operation. If DCM operation is

required, the off time of the LS-FET in CCM

is 37ns. It is difficult to enter DCM at light load.

Using a smaller inductor can improve this and

make it easier to enter DCM.

Enable (EN)

When VIN1 is greater than the under-voltage

lockout (UVLO) threshold (typically 2.7V), the

regulators or the load switch can be enabled by

pulling its EN pins above the EN UVLO

threshold. Leave the EN pins floating or pull the

EN pins down to ground to disable the

corresponding channel. There is an internal 2M

Ω

resistor from the EN pins to ground. There is a

delay of about 100µs for VIN1 and VIN2 enable

start-up. The VIN3 enable start-up delay is

shorter (around 70µs).

Soft Start (SS) and Output Discharge

The MP5403B has a built-in soft start (SS) that

ramps up the output voltage at a controlled slew

rate to prevent overshooting at start-up for both

step-down regulators and the load switch. The

soft-start time is set to around 0.35ms. When the

regulators are disabled, the internal discharge