MP9459 – SYNCHRONOUS, STEP-DOWN CONVERTER W/ INTERNAL MOSFETS

MP9459 Rev. 1.0

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11

1/13/2020

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VSS overrides VREF, so the error amplifier uses

VSS as the reference. When VSS exceeds VREF,

the error amplifier uses VREF as the reference.

The SS time is set internally to 2.5ms.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

The MP9459 has valley current limit control.

The inductor current is monitored when the LS-

FET is on. When the sensed inductor current

reaches the valley current limit, the low-side

limit comparator turns over, and the device

enters over-current protection (OCP) mode.

The HS-FET waits until the valley current limit is

removed before turning on again. Meanwhile,

the output voltage drops until VFB falls below the

under-voltage (UV) threshold (about 75% of

VREF). Once UV is triggered, the MP9459 enters

hiccup mode to periodically restart the part.

During OCP, the device attempts to recover

from the over-current fault with hiccup mode. In

hiccup mode, the chip disables the output

power stage, discharges the soft start, and then

automatically attempts to soft start again. If the

over-current condition still remains after soft

start ends, the device repeats this operation

cycle until the over-current condition is removed.

Then the output rises back to the regulation

level. OCP is a non-latch protection.

Pre-Biased Start-Up

The MP9459 is designed for monotonic start-up

into pre-biased loads. If the output is pre-biased

to

a

certain

voltage

during

start-up,

the

bootstrap (BST)

voltage

is refreshed and

charged, and the soft-start voltage is also

charged. If the BST voltage exceeds its rising

threshold voltage and the soft-start voltage

exceeds the sensed output voltage at FB, the

part begins working normally.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection with a rising

threshold of 2.2V and a hysteresis of 150mV.

VIN regulates the bootstrap capacitor voltage

internally through D1, M1, C3, L1, and C2 (see

Figure 3). If VIN - VSW exceeds 3.3V, U1

regulates M1 to maintain a 3.3V BST voltage

across C3.

Figure 3: Internal Bootstrap Charger

Start-Up and Shutdown

If both VIN and EN exceed their respective

thresholds, the chip starts up. 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 circuits.

Three events can shut down the chip: EN going

low, VIN going low, and thermal shutdown. The

shutdown procedure starts by blocking the

signaling path to avoid any fault triggering. The

internal supply rail is then pulled down.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds

150°C, the entire chip shuts down. When the

temperature falls below its lower threshold

(typically 130°C), the chip is enabled again.