MP1471A

– SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1471A Rev. 1.0

www.MonolithicPower.com

11

8/18/2013

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current to less than 100

μA.

For example, with 12V connected to Vin,

Connecting the EN pin directly to a voltage

source without any pullup resistor requires

limiting the amplitude of the voltage source to ≤

6V to prevent damage to the Zener diode.

Figure 4: 6.5V Zener Diode

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP1471A UVLO comparator monitors the

output voltage of the internal regulator, VCC.

The UVLO rising threshold is about 4.15V while

its falling threshold is consistently 3.8V.

Internal Soft-Start

Soft-start prevents the converter output voltage

from overshooting during startup. When the

chip starts, the internal circuit generates a soft-

start voltage (SS) that ramps up from 0V to

1.2V:

When

SS

falls

below

the

internal

reference (REF), SS overrides REF so that the

error amplifier uses SS as the reference; when

SS exceeds REF, the error amplifier resumes

using REF as its reference. The SS time is

internally set to 1.5ms.

Over-Current-Protection and Hiccup

The MP1471A has a cycle-by-cycle over-

current limit for when the inductor current peak

value exceeds the set current-limit threshold.

First, when the output voltage drops until FB

falls below the Under-Voltage (UV) threshold

(typically 140mV) to trigger a UV event, the

MP1471A enters hiccup mode to periodically

restart the part. This protection mode is

especially useful when the output is dead-

shorted to ground. This greatly reduces the

average short-circuit current to alleviate thermal

issues and to protect the regulator. The

MP1471A exits hiccup mode once the over-

current condition is removed.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds

150°C, it shuts down the whole chip. 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 2.2V and a hysteresis of

voltage internally through D1, M1, R4, C4, L1

will regulate M1 to maintain a 5V BST voltage

across C4.

Figure 5 : Internal Bootstrap Charging Circuit

Start-Up and Shutdown

thresholds, the chip starts. 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 circuits.

Three events can shut down the chip: EN low,

procedure

starts

by

initially

blocking

the

signaling path to avoid any fault triggering. The

COMP voltage and the internal supply rail are

then pulled down. The floating driver is not

subject to this shutdown command.