MP9459 – SYNCHRONOUS, STEP-DOWN CONVERTER W/ INTERNAL MOSFETS

MP9459 Rev. 1.0

www.MonolithicPower.com

10

1/13/2020

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© 2020 MPS. All Rights Reserved.

OPERATION

The MP9459 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

Constant-on-time (COT) control provides fast

transient response and eases loop stabilization.

At the beginning of each cycle, the high-side

MOSFET

(HS-FET)

turns

on

when

the

feedback

voltage

(VFB)

drops

below

the

reference voltage (VREF). The HS-FET turns on

for a fixed interval determined by the one-shot

on timer. The on time is determined by both the

output voltage and input voltage to maintain a

constant switching frequency over the input

voltage range.

After the on period elapses, the HS-FET turns

off until the next period. By repeating this

operation, the converter regulates the output

voltage.

Continuous conduction mode (CCM) occurs

when the output current is high and the inductor

current is above 0A. The low-side MOSFET

(LS-FET) turns on when the HS-FET is off to

minimize conduction loss. If both the HS-FET

and LS-FET are turned on at the same time,

there is a dead short between the input and

GND. This is called shoot-through. To avoid

shoot-through,

a

dead

time

is

generated

internally between the HS-FET off and LS-FET

on period, or vice versa.

When the MP9459 works in pulse-frequency

modulation

(PFM)

mode

during

light-load

operation, the device reduces the switching

frequency

automatically

to

maintain

high

efficiency, and the inductor current drops to

almost 0A. When the inductor current reaches

0A, the low-side driver goes into tri-state (Hi-Z).

Then the output capacitors discharge slowly to

GND through resistors R1 and R2. When VFB

drops below VREF, the HS-FET turns on. This

operation greatly improves device efficiency

when the output current is low.

Light-load operation is also called skip mode

because the HS-FET does not turn on as

frequently

as

it

does

during

heavy-load

conditions. The HS-FET turn-on frequency is a

function of the output current. As the output

current

increases,

the

current

modulator

regulates over a shorter period, and the

HS-FET turns on more frequently. The switching

frequency also increases. The output current

(IOUT) reaches the critical level when the current

modulator time is zero. IOUT can be calculated

with Equation (1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2 L f

V





 



(1)

The device reverts to pulse-width modulation

(PWM) mode once the output current exceeds

the

critical

level.

Afterward,

the

switching

frequency remains fairly constant over the

output current range.

Enable (EN) Control

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 it off. An

internal 1M

Ω resistor from EN to GND allows

EN to be floated to shut down the chip.

EN is clamped internally using a 2.8V series

Zener diode (see Figure 2). Connect the EN

input to VIN through a pull-up resistor to limit

the EN input current below 100

μA and prevent

damage to the Zener diode.

For example, when connecting 12V to VIN, then

RPULLUP ≥ (12V - 2.8V) / (100kΩ + 35kΩ) = 68µA.

EN

Logic

1M

Ω

EN

GND

35k

Ω

2.8V

Figure 2: Zener Diode between EN and GND

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP9459

’s UVLO comparator monitors the

output voltage of the internal regulator (VCC).

The UVLO rising threshold is about 4V, and its

falling threshold is 3.67V.

Internal Soft Start (SS)

Soft start (SS) prevents the converter output

voltage

from

overshooting

during

start-up.

When the chip starts up, the internal circuitry

generates an SS voltage (VSS) that ramps up

from 0V to 1.2V. When VSS drops below VREF,