MP9928—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER

MP9928 Rev. 1.0

www.MonolithicPower.com

18

5/20/2016

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HS-FET is turned off, and the LS-FET is turned

on and keeps on until the reverse current limit is

triggered, after LS-FET is turned off, inductor

current will increase to 0. The LS-FET will be

turned on again next clock cycle. MP9928 works

at

discharge

mode

until

the

over-voltage

condition is cleared.

Enable

The MP9928 has a dedicated enable control pin.

It uses a bandgap generated precision threshold

of 1.22V. By pulling it high or low, the IC can be

enabled

or

disabled.

To

disable

the

part,

EN/SYNC must be pulled low for at least 15µs.

Tie EN to VIN through a resistor divider R16 and

R17 to program the VIN start up threshold (see

Figure 3). The EN threshold is 1.08V (falling

edge), so the VIN falling UVLO threshold is

1.08V x (1+ R16/R17).

In high input design, EN pin voltage should not

be greater than 50V.

EN

VIN

R16

R17

Figure 3: EN Resistor Divider

Synchronize

The MP9928 can be synchronized to an external

clock range from 100kHz up to 1000kHz through

EN/SYNC pin. The internal clock rising edge is

synchronized to the external clock rising edge.

The pulse width (both on and off) of external

clock signal should be no less than 100ns.

Under-Voltage Lockout

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

input supply voltages. The MP9928 UVLO rising

threshold is about 4.5V while its falling threshold

is about 3.7V.

Thermal Protection

The purpose of thermal protection is to prevent

damage in the IC by allowing exceptive current to

flow

and

heating

the

junction.

The

die

temperature is internally monitored until the

thermal limit is reached. When the silicon die

temperature is higher than 170°C, it shuts down

the whole chip. When the temperature is lower

than its lower threshold, typically 150°C, the chip

is enabled again.

Start-Up and Shutdown

If both VIN and EN are higher than their

respective thresholds, the chip starts up. The

reference block starts first, generating stable

reference voltages and currents. And then the

internal regulator is enabled. The regulator

provides stable supply for the remaining circuitry.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In the shutdown

procedure, the signal path is firstly blocked to

avoid any fault triggering. The COMP voltage and

the internal supply rail are then pulled down. The

floating driver is not subjected to this shutdown

command.

Pre-Bias Start-Up

For MP9928, at startup, If SS<FB, which means

output has pre-bias voltage, neither TG nor BG

would be turned on until SS is greater than FB.