MP5402M – STEP-DOWN CONVERTER WITH SMART DUAL USB CHARGING PORTS

MP5402M Rev.1.0

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12

10/10/2015

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alleviates thermal issues, and protects the

regulator. The MP5402M exits hiccup mode

once the over-current condition is removed.

Buck Output Over-Voltage Protection (OVP)

The

MP5402M

has

output

over-voltage

protection (OVP). If the output is higher than 6V,

the high-side switch stops switching, and the

low-side switch turns on to discharge the output

voltage until the output decreases to 5.75V. The

chip then returns to normal operation. If the

output over voltage cannot be discharged to

5.75V, the low-side switch turns off after the

inductor current reaches a negative current limit.

The low-side switch turns on again when the

next clock cycle begins.

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

exceeds 5V, U1 regulates M1 to maintain a 5V

BST voltage across C4.

D1

M1

BST

C4

L1

SW

U1

5V

C2

Figure 3: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

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,

signaling path is blocked to avoid any fault

triggering, and the COMP voltage and the

internal supply rail are pulled down. The floating

driver is not subject to this shutdown command.

Buck Output Discharge

The buck portion of the device involves a

discharge function that provides a resistive

discharge path for the external output capacitor.

The function is active when the part is disabled

(input voltage is under UVLO, EN off) and is

done in a very limited time. After VCC

discharges below 1V, the buck output discharge

resistor disconnects.

USB CURRENT-LIMIT SWITCH

Current-Limit Switch

The MP5402M integrates two USB current-limit

switches. It provides a built-in soft-start circuitry,

which controls the rising slew rate of the output

voltage to limit the inrush current and voltage

surges.

When the load current reaches the current-limit

threshold (2.75A, typically), the USB power

MOSFET switches into foldback current-limit

mode, 70% of the current limit (see Figure 4). If

the over-current limit condition lasts for more

than 3ms, the USB channel enters hiccup mode

with a 3ms on time and a 8.5s off time.

Figure 4: Over-Current Limit

load current does not exceed 70% of the current

limit to avoid triggering a foldback current limit

and start-up failure.

Output Line Drop Compensation

The MP5402M can compensate an output-

voltage drop—such as high impedance caused

by a long trace—to keep a fairly constant 5V

load-side voltage.

The internal comparator compares the current-

sense output voltage of the two current-limit

switches and uses the larger current-sense

output voltage to compensate the line drop

voltage.