MP5408M – 36V, 6A, STEP-DOWN CONVERTER W/ DUAL USB CHARGING PORTS

MP5408M Rev. 1.1

www.MonolithicPower.com

12

8/14/2017

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© 2017 MPS. All Rights Reserved.

Table 1: Recommended Resistor Values for

Typical Switching Frequency

(kΩ)

(kHz)

0

235

66.5

350

NS

450

45.8

500

22.3

1000

14.6

1500

9.53

2200

Two internal comparators monitor FREQ's logic

voltage to enable FREQ to float or short to GND.

During power-up, there is another internal

source current on FREQ. The frequency is

locked at 450kHz when a voltage greater than

2V and a current greater than 8µs is sensed on

FREQ. The frequency is locked at 235kHz

when a voltage less than 0.1V and a current

greater than 8µs is sensed on FREQ. Leave

FREQ floating or connect FREQ to VCC to

achieve the 450kHz default switching frequency.

Short FREQ to ground to achieve a 235kHz

frequency (see Figure 4).

Figure 4: Switching Frequency Functional Block

Electromagnetic Interference (EMI)

Based on MPS’s flip-chip package technology,

the MP5408M integrates two sets of EMI filters

to reduce the input path’s parasitic inductance

dramatically. Simultaneously, the input pin and

GND

pin

are

assigned

on

two

sides

symmetrically, partially cancelling the magnetic

field inside those two filters (see Figure 5). The

magnetic field in the overlap area (A1) is

partially cancelled.

Using the above technique, the radiated

emission performance is improved greatly.

Figure 5: EMI Reduction

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 5V, and

its falling threshold is 4.3V.

Internal Soft Start (SS)

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 5V. When SS is lower than REF, the error

amplifier uses SS as the reference. When SS is

higher than REF, the error amplifier uses REF

as the reference. The SS time is set to 2ms

internally. If the output of the MP5408M 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.

Forced CCM Operation

The MP5408M works in forced continuous

conduction mode (CCM) continuously. The

MP5408M operates in a fixed switching

frequency regardless of whether it is operating

in light load or full load. The advantage of CCM

is the controllable frequency, smaller output

ripple, and sufficient bootstrap charge time, but

it also has low efficiency at light-load condition.

A proper inductance should be selected to

avoid triggering the low-side switch's negative

current limit (typically 2A, from SW to GND). If

the negative current limit is triggered, the low-

side switch turns off, and the high-side switch

turns on when the internal clock comes around.

Buck Over-Current Protection (OCP)

The MP5408M has a cycle-by-cycle over-

current limit when the inductor peak current

exceeds the current-limit threshold, and the FB

voltage drops below the under-voltage (UV)