MP4541

– 80V, 0.8A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4541 Rev. 1.0

MonolithicPower.com

12

5/28/2021

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© 2021 MPS. All Rights Reserved.

OPERATION

The MP4541 is a synchronous step-down

switching

converter

with

integrated

power

MOSFETs. It provides high efficiency and

constant-on-time (COT) control for fast loop

response and easy loop stabilization. The

MP4541 features a wide 8V to 80V input voltage

range, internal soft-start (SS) control, and

precise current limiting. Its ultra-low quiescent

current (IQ) enables high efficiency under light-

load conditions.

Constant-On-Time (COT) Control

The MP4541 includes constant-on-time (COT)

control mode. At the beginning of each cycle, the

high-side MOSFET (HS-FET) turns on for a fixed

amount of time when the FB

pin’s voltage (VFB)

drops below the reference voltage (VREF). This

fixed on time is determined by a one-shot on-

timer. The on time is determined by both the

output voltage (VOUT) and input voltage (VIN) to

maintain a constant switching frequency (fSW)

across the entire input voltage range.

Once the on period is complete, the HS-FET

turns off until the next period. By repeating this

operation, the converter can regulate VOUT.

The integrated low-side MOSFET (LS-FET)

turns on when the HS-FET is off to minimize

conduction loss. A dead short occurs between

the VIN pin and GND pin if the HS-FET and LS-

FET are turned on at the same time. This is

called shoot-through. To avoid shoot-through, a

dead time is generated internally between the

HS-FET and LS-FET on and off periods. The

dead time occurs between the HS-FET off time

and the LS-FET on time, or vice versa.

Heavy-Load Operation

The MP4541 operates in continuous conduction

mode (CCM) when the output current (IOUT) is

high and the inductor current is above 0A. In

CCM, the HS-FET turns on, then turns off once

the on period is complete. Once the HS-FET

turns off, the LS-FET turns on to conduct the

inductor current.

Pulse-width modulation (PWM) operation occurs

when fSW remains constant while the part is in

CCM.

Light-Load Operation

The MP4541 can work in pulse-skip mode (PSM)

under light-load conditions. In PSM, the LS-FET

goes into tri-state (Hi-Z) when the inductor

current drops close to 0A, and the output

capacitors discharge slowly to GND through the

FB

pin’s feedback resistor. If VOUT drops and the

internal EA output voltage rises, the MP4541

starts the next switching cycle by turning on the

HS-FET. The MP4541 automatically reduces fSW

and IQ when the device is not switching. This

improves the device

’s efficiency when IOUT is low.

When the MP4541 is in PSM under light-load

conditions, the HS-FET does not turn on as

frequently

as

it

does

under

heavy-load

conditions. The frequency at which the HS-FET

turns on is a function of IOUT. As IOUT increases,

the HS-FET turns on more frequently. In turn, fSW

also increases. IOUT exceeds the upper boundary

threshold when the valley inductor current

reaches 0A. IOUT can be calculated with Equation

(1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2 L f

V





 



(1)

Where fSW is the switching frequency.

The MP4541 returns to CCM once IOUT exceeds

the upper boundary threshold. After this point,

fSW remains constant across the entire IOUT range.

Switching Frequency (fSW)

Selecting a switching frequency (fSW) is a tradeoff

between efficiency and component size. Low-

frequency operation increases efficiency by

reducing MOSFET switching loss, but requires a

larger inductance and capacitance to maintain

the low output voltage ripple.

The MP4541 offers configurable COT control.

When the FREQ pin is connected to the GND pin

via a resistor (RFREQ), VIN is internally fed-forward

to the one-shot on-timer. When the MP4541 is in

steady state operation in CCM, the duty ratio is

set to VOUT / VIN. Therefore, fSW remains constant

across the entire input voltage range. fSW can be

estimated with Equation (2):

6

SW

FREQ

10

f

+57 (kHz)

34.5 R

(k

Ω)



 





(2)