MP8712 – 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8712 Rev.1.01

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16

3/28/2017

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usually cannot be used as output capacitors.

The

MP8712

has

built-in

internal

ramp

compensation to ensure that the system is

stable even without the help of the output

capacitor’s ESR. A pure ceramic capacitor

solution can reduce output ripple, total BOM

cost, and board area significantly.

VCC Regulator

A 3.5V internal regulator powers most of the

internal

circuitries.

A

470nF

decoupling

capacitor is needed to stabilize the regulator

and reduce ripple. This regulator takes the VIN

input and operates in the full VIN range. After

EN is pulled high and VIN is greater than 3.5V,

the output of the regulator is in full regulation.

When VIN is lower than 3.5V, the output

voltage decreases and follows the input voltage.

A 0.47μF ceramic capacitor is required for

decoupling purposes.

Error Amplifier (EA)

The error amplifier (EA) compares the FB

voltage against the internal 0.6V reference

(REF) and outputs a PWM signal. The

optimized

internal

ramp

compensation

minimizes the external component count and

simplifies the control loop design.

Enable (EN)

EN is a digital control pin that turns the

regulator on and off. Drive EN high to turn on

the regulator; drive EN low to turn off the

regulator. An internal

1.5MΩ resistor is

connected from EN to ground. EN can operate

with an 18V input voltage, which allows EN to

be connected to VIN directly for automatic start-

up.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP8712 UVLO comparator monitors both

the input voltage (VIN) and the output voltage

(VOUT) of the VCC regulator. The MP8712 is

active when both voltages exceed the UVLO

rising threshold.

Soft Start (SS) and Pre-Bias Start-Up

Soft start prevents the converter output voltage

from overshooting during start-up. When the

chip starts up, the internal circuitry generates a

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

VCC. When SS is lower than REF, the error

amplifier uses SS as the reference. When SS is

higher than REF, the error amplifier uses REF

as the reference.

The approximate typical soft-start time can be

calculated with Equation (2):

()

(

)

()

7

ref

ss

ss

VV

C

nF

tms

A







(2)

If the output of the MP8712 is pre-biased to a

certain voltage during start-up, the IC disables

the switching of the high-side and low-side until

the voltage on the internal soft-start capacitor

exceeds the sensed output voltage at FB.

Over-Current Protection (OCP)

The MP8712 has hiccup, cycle-by-cycle, and

over-current limiting control. The current-limit

circuit employs both the high-side current limit

and the low-side (valley) current-sensing

LS-FET as a current-sensing element for the

valley current limit. If the magnitude of the high-

side current-sense signal is above the current-

limit threshold, the PWM on pulse is terminated,

and the low side is turned on. Afterward, the

inductor current is monitored by the voltage

between GND and SW. GND is used as the

positive current sensing node, so GND should

be connected to the source terminal of the

bottom MOSFET. PWM is not allowed to initiate

a new cycle before the inductor current falls to

the valley threshold.

After the cycle-by-cycle over-current limit

occurs, the output voltage drops until VOUT is

below

the

under-voltage

(UV)

threshold,

typically 60% below the reference. Once UV is

triggered, the MP8712 enters hiccup mode to

restart the part periodically. This protection

mode is especially useful when the output is

dead-shorted to ground. The average short-

circuit current is reduced greatly to alleviate

thermal issues and protect the regulator. The

MP8712 exits hiccup mode once the over-

current condition is removed.