MP8715 – 4A, 21V, 500kHz, STEP DOWN CONVERTER WITH 100% DUTY CYCLE OPERATION

MP8715 Rev. 1.01

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11

6/13/2013

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internal clock rising edge synchronized to the

external clock rising edge.

Vin

EN/Sync

Vcc

Vout

CLK

5us

Foldback

500kHz

External CLK

0.625*Vout_set

2ms

1ms

VCC_Rising

Vout_set

Figure 4—Startup Sequence Using External

Sync Clock Signal

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The MP8715 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 3.1V.

Power OK Indicator

When the FB is below 0.85VFB, the POK pin will

be internally pulled low. When the FB is above

0.9VFB , the POK becomes an open-drain output.

If POK function is not used, it can be left open.

External 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 3.5V. When it is lower

than the internal FB reference (REF), SS

overrides REF so the error amplifier uses SS as

the reference. When SS is higher than REF, REF

regains control. The SS time can be set by

external decoupled cap. The soft-start time can

be calculated as below:

SS

5

t(ms)

Vref(V) C (nF)

5A

=

×

μ

To reduce the susceptibility to noise, do not leave

SS pin open. Use a capacitor with small value if

you do not need soft function.

Over-Current-Protection and Hiccup

The MP8715 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 MP8715

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 MP8715 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 130

°C, the chip is

enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is 2.5V with a hysteresis of

150mV. The bootstrap capacitor voltage is

regulated internally by VIN through D1, D2, M3,

C4, L1 and C2 (Figure 5). If (VIN-VSW) is more

than 5V, U2 will regulate M3 to maintain a 5V

BST voltage across C4.

D2

Charge

Pump

Figure 5—Internal Bootstrap Charging Circuit

Startup and Shutdown

If both VIN and EN are higher than their

appropriate thresholds, the chip starts. The