MP8768 – 8A, 28V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFET

MP8768 Rev. 1.0

www.MonolithicPower.com

7

12/12/2013

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OPERATION

The MP8768 is a high frequency synchronous

rectified step-down switch mode converter with a

built in internal high side power MOSFET. It

offers a very compact solution to achieve 8A

continuous output current over a wide input

supply range with excellent load and line

regulation.

The MP8768 operates in a fixed frequency, peak

current control mode to regulate the output

voltage. A PWM cycle is initiated by the internal

clock. The integrated high-side power MOSFET

is turned on and remains on until its current

reaches the value set by the COMP voltage.

When the power switch is off, it remains off until

the next clock cycle starts. If, in 90% of one PWM

period, the current in the power MOSFET does

not reach the COMP set current value, the power

MOSFET will be forced to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal 0.8V reference (REF) and

outputs a current proportional to the difference

between the two. This output current is then used

to charge or discharge the internal compensation

network to form the COMP voltage, which is used

to control the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component counts and

simplifies the control loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes the

VIN input and operates in the full VIN range.

When VIN is greater than 5.0V, the output of the

regulator is in full regulation. When VIN is lower

than 5.0V, the output decreases. Since this

internal regulator provides the bias current for the

bottom gate driver that requires significant

amount of current depending upon the external

MOSFET selection, a 1uF ceramic capacitor for

decoupling purpose is required.

Enable/Sync Control

The MP8768 has a dedicated Enable/Sync

control pin (EN/SYNC). By pulling it high or low,

the IC can be enabled and disabled by EN. Tie

EN to VIN for automatic start up. To disable the

part, EN must be pulled low for at least 5µs.

The MP8768 can be synchronized to external

clock range from 300KHz up to 2MHz through

the EN/SYNC pin. The internal clock rising edge

is synchronized to the external clock rising edge.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The MP8768 UVLO comparator

monitors the output voltage of the internal

regulator, VCC. The UVLO rising threshold is

about 4.0V while its falling threshold is a

consistent 3.2V.

Internal Soft-Start

The soft-start is implemented to prevent the

converter output voltage from overshooting

during startup. When the chip starts, the internal

circuitry generates a soft-start voltage (SS)

ramping up from 0V to 1.2V. When it is lower

than the internal reference (REF), SS overrides

REF so the error amplifier uses SS as the

reference. When SS is higher than REF, REF

regains control.

Over-Current-Protection and Hiccup

The MP8768 has cycle-by-cycle over current limit

when the inductor current peak value exceeds

the set current limit threshold. Meanwhile, output

voltage starts to drop until FB is below the Under-

Voltage (UV) threshold, typically 30% below the

reference. Once a UV is triggered, the MP8768

enters hiccup mode to periodically restart the part.

This protection mode is especially useful when

the output is dead-short to ground. The average

short circuit current is greatly reduced to alleviate

the thermal issue and to protect the regulator.

The MP8768 exits the hiccup mode once the

over current condition is removed.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip

from

operating

at

exceedingly

high

temperatures. When the silicon die temperature

is higher than 150

°C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 140

°C, the chip is

enabled again.