MP5408M – 36V, 6A, STEP-DOWN CONVERTER W/ DUAL USB CHARGING PORTS

MP5408M Rev. 1.1

www.MonolithicPower.com

11

8/14/2017

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OPERATION

BUCK CONVERTER SECTION

The

MP5408M

integrates

a

monolithic,

synchronous, rectified, step-down, switch-mode

converter with internal power MOSFETs and

two USB current-limit switches with charging-

port auto-detection. The MP5408M offers a

compact

solution

that

achieves

6A

of

continuous output current with excellent load

and line regulation over a wide input supply

range.

The MP5408M operates in a fixed-frequency,

peak-current-mode control to regulate the

output voltage. The internal clock initiates the

pulse-width modulation (PWM) cycle, which

turns on the integrated high-side power

MOSFET (HS-FET). The HS-FET remains on

until its current reaches the value set by the

is off, it remains off until the next clock cycle

begins. If the duty cycle reaches 95% (450kHz

switching frequency) in one PWM period, the

current in the power MOSFET does not reach

the COMP-set current value, and the power

MOSFET turns off.

Error Amplifier (EA)

The error amplifier (EA) compares the internal

feedback voltage against the internal reference

power MOSFET current. The optimized, internal

compensation network minimizes the external

component count and simplifies the control loop

design.

Internal VCC Regulator

The 4.5V internal regulator powers most of the

exceeds 4.5V, the output of the regulator is in

1µF ceramic decoupling capacitor.

After the buck output starts up, the internal VCC

LDO output is biased by the buck output

through a Schottky diode.

Enable (EN) Control

The MP5408M has an enable (EN) control pin.

An internal 8μA pull-up current allows EN to be

floated for automatic start-up. Pull EN high or

float EN to enable the IC. Pull EN low to disable

the IC.

EN is clamped internally using a 7.6V series

Zener diode and a 10V break-down voltage of

the ESD cell (see Figure 2).

enhance the EN pull-up current ability. This

requires limiting the EN voltage below 10V or

limiting EN input current below 500μA if the EN

pull-up voltage is larger than 10V.

Figure 2: Zener Diode between EN and GND

Setting the Switching Frequency

Connect a resistor from FREQ to ground to set

the switching frequency (see Table 1). The

value of the frequency can be calculated

approximately with Equation (1):





Ω

FREQ

1000000

FREQ(kHz)

42.5 R

(K ) 53.7

(1)

200

700

1200

1700

2200

0

20

40

60

80 100 120