MP8771

– 17V, 10A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8771 Rev. 1.0

www.MonolithicPower.com

16

1/15/2018

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© 2018 MPS. All Rights Reserved.

Figure 5: PG Clamped Voltage vs. Pull-Up

Current

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

The MP8771 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 MP8771 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 MP8771 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

regulates

the

bootstrap

capacitor

voltage

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.

0

0.2

0.4

0.6

0.8

1

1.2

0

1

2

3

4

5

PULL UP CURRENT (mA)

PG Clamped Voltage vs. Pull Up Current