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

MP5408M Rev. 1.1

www.MonolithicPower.com

13

8/14/2017

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threshold (typically 50% below the reference).

Once UV is triggered, the MP5408M enters

hiccup mode to restart the part periodically.

This protection mode is especially useful when

the output is dead-shorted to ground. This

reduces the average short-circuit current greatly,

alleviates thermal issues, and protects the

regulator. The MP5408M exits hiccup mode

once the over-current condition is removed.

Buck Output Over-Voltage Protection (OVP)

The

MP5408M

has

output

over-voltage

protection (OVP). If the output is higher than

5.85V, the high-side switch stops turning on.

The low-side switch turns on to discharge the

output voltage until the output decreases to

5.7V, and then the chip resumes normal

operation.

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

5). BST capacitor C4 voltage will be charged

up quickly by VCC through M1. The 2.5μA input

to BST current source also can charge the BST

capacitor when low-side switch doesn't turn-on.

Figure 6: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

If both IN and EN exceed their respective

thresholds, the chip is enabled. The reference

block starts first, generating a stable reference

voltage and current, and then the internal

regulator is enabled. The regulator provides a

stable supply for the remaining circuitries.

Three events can shut down the chip: EN low, IN

low, and thermal shutdown. In shutdown, the

signaling path is blocked to avoid any fault

rail are pulled down. The floating driver is not

subject to this shutdown command.

Buck Output Impedance

The Buck doesn't involve output discharge

function during EN shutdown. After EN

shutdown, there is only two feedback resistor

connected to the OUT pin, which has a typical

resistance of 175kΩ in total. Refer to the block

diagram.

USB CURRENT-LIMIT SWITCH SECTION

Over-Current Protection (OCP) and Hiccup

The MP5408M integrates two USB current-limit

switches. The MP5408M provides built-in soft-

start circuitry which controls the rising slew rate

of the output voltage to limit inrush current and

voltage surges.

When the load current reaches the current-limit

threshold the USB power MOSFET works in

constant current-limit mode (see Figure 7). If

the over-current limit condition lasts more than

corresponding USB channel enters hiccup

mode with 5ms of on-time and 2s of off-time.

Another USB channel works normally.

Figure 7: Over-Current Limit

After the soft start finishes, if the USB output

voltage is lower than 3.5V and lasts longer than

50µs, the MP5408M enters hiccup without

having to wait 5ms (see Figure 8). This can

prevent abnormal thermal rise during the

constant resistor (CR) load over-current case.