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

MP9928 Rev. 1.0

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17

5/20/2016

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regulated to 5V. Using the VCC2 power supply

allows the VCC1 power to be derived from a

high-efficiency external source, such as one of

the MP9928’s switching regulator outputs.

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 input voltages. This output

current is then used to charge or discharge the

external compensation network to form the

COMP voltage, which is used to control the

power

MOSFET

current.

Adjusting

the

compensation network from COMP pin to GND

could optimize the control loop for good stability

or fast transient response.

Current Limit Function

There are three fixed options for current limit

setting: When ILIM connects to GND, the current

limit sense voltage is set to 25mV; when ILIM

connects to VCC1, the current limit sense voltage

is set to 50mV; when ILIM pin floats, the current

limit sense voltage is set to 75mV.

When the peak value of the inductor current

exceeds

the

set

current

limit

threshold,

meanwhile, output voltage starts to drop until FB

is 62.5% of the reference. MP9928 enters hiccup

mode to periodically restart the part. Meanwhile,

the frequency would be lowered when FB<0.5V.

This protection mode is especially useful when

the output is dead-shorted to ground. The

average short-circuit current is greatly reduced to

alleviate the thermal issues. The MP9928 exits

the hiccup mode once the over-current condition

is removed.

Low Dropout Operation

At low dropout mode, the MP9928 is designed to

operate at HS max duty on mode as long as the

voltage across BST - SW is greater than 3.05V,

this improves dropout. When the voltage from

BST to SW drops below 3.05V, an under-voltage

lockout (UVLO) circuit turns off the high-side

MOSFET (HS-FET), and at the same time, the

low-side MOSFET (LS-FET) turns on to refresh

the BST capacitor. After the BST capacitor

voltage is re-charged, the HS-FET turns on again

to regulate the output. Since the BST capacitor

voltage is greater than 3.05V, the HS-FET can

remain on for more switching cycles than are

required to refresh the BST capacitor, thus

increasing

the

effective

duty

cycle

of

the

switching regulator. The low dropout operation

makes the MP9928 suitable for application such

as automotive cold-crank.

Power Good Function

The MP9928 includes an open-drain power good

output that indicates whether the regulator’s

output is within about ±10% of its nominal value.

When the output voltage falls outside this range,

the PG output is pulled to low. It should be

connected to a voltage source of no more than

5V through a resistor (e.g., 100k

Ω). The PG

delay time is 25µs.

PG pin has self-driving capability, if MP9928 is

off and PG pin is pulled up to another DC power

source through a resistor, the PG pin can also be

pulled low by self-driving circuit.

Soft Start

The soft start (SS) is implemented to prevent the

converter

output

voltage

from

overshooting

during startup. When the chip starts, the internal

circuitry generates a soft-start voltage ramping

up from 0V to 0.8V. When it is lower than the

internal reference (REF), SS voltage overrides

REF, so the error amplifier uses SS voltage as

the reference. When SS voltage is higher than

REF, REF regains control.

An external capacitor connected from SS to

SGND is charged from an internal 4

μA current

source, producing a ramped voltage. The soft-

and can be calculated by below formula:

( )

( )

( )

( )

μA

I

V

V

nF

C

ms

t

SS

REF

SS

SS

×

=

the 4

μA SS charge current. There is no internal

SS capacitor.

SS

will

be

reset

when

a

fault

protection

happened

except

for

output

over

voltage

protection.

Output Over-Voltage Protection

MP9928 output voltage is monitored by FB

voltage. If FB voltage is typically 10% higher than

the reference, it’ll trigger OVP. Once it triggers

OVP, MP9928 will go into discharge mode, the