MP8774

– 18V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8774 Rev. 1.0

www.MonolithicPower.com

17

1/15/2018

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PULL UP CURRENT (mA)

PG Clamped Voltage vs. Pull Up Current

Figure 5: PG Clamped Voltage vs. Pull-Up

Current

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

The MP8774 has a valley-limit control. The LS-

FET monitors the current flowing through the

LS-FET. The HS-FET waits until the valley

current limit is removed before turning on again.

below

the

under-voltage

(UV)

threshold

(typically 50% below the reference). Once UV is

triggered, the MP8774 enters hiccup mode to

restart the part periodically.

During

over-current

protection

(OCP),

the

device attempts to recover from the over-

current fault with hiccup mode. This means that

the chip disables the output power stage,

discharges

the

soft-start

capacitor,

and

attempts to soft start again automatically. If the

over-current condition still remains after the soft

start ends, the device repeats this operation

cycle until the over-current condition disappears,

and then the output rises back to the regulation

level. OCP is a non-latch protection.

Pre-Bias Start-Up

The MP8774 is designed for monotonic start-up

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

to a certain voltage during start-up, the BST

voltage is refreshed and charged, and the

voltage on the soft-start capacitor is charged as

well. If the BST voltage exceeds its rising

threshold voltage and the soft-start capacitor

voltage exceeds the sensed output voltage at

FB, the part begins working normally.

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.

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 1.7V and a hysteresis of 150mV.

internally through D1, M1, R4, C4, Lo, and Co

regulates M1 to maintain a 3.3V BST voltage

across

C4.

The

BST

resistor

(R4)

is

recommended to be

less than 4.7Ω.

Vin

M1

C4

SW

VOUT

Co

3.3V

Lo

D1

R4

U2

Figure 6: Internal Bootstrap Charger

Start-Up and Shutdown Circuit

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 low,

procedure starts by blocking the signaling path

initially to avoid any fault triggering. The internal

supply rail is then pulled down.