MPM3686 – 18V 20A STEP-DOWN POWER MODULE

MPM3686 Rev. 1.02

www.MonolithicPower.com

13

10/13/2016

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Table 1 shows the switching frequency with

different common output voltages:

Vo (V)

fs(KHz)

1400

1.2

500

1.5

600

1.8

700

Table 1

If a switching frequency other than those listed in

Table 1 is desired, an external frequency set

resistor can be connected as shown in figure 5:

Figure 5

The switching frequency can be estimated

through equation (4) as follows:

6

SW

FREQ

IN

DELAY

IN

OUT

10

F(kHz)

6.1 R

(k )

V(V)

T(ns)

V(V) 0.4

V

(V)









(4)

5ns.

Typically, the MPM3686 is set to 200kHz to

1MHz applications. Thanks to its monolithic

structure, the MPM3686 is optimized to operate

at high switching frequencies at high efficiency.

High switching frequencies allow for physically

smaller LC filter components to reduce the PCB

footprint.

Configuring the EN Control

The power module turns on when EN goes high;

conversely it turns off when EN goes low. Do not

float the pin.

For automatic start-up, pull the EN pin up to input

voltage through a resistive voltage divider.

from the IN pin to the EN pin) and the pull-down

determine the automatic start-up voltage:

UP

DOWN

IN START

DOWN

(R

R

)

V1.5

(V)

R







(5)

To reduce noise, add a 10nF ceramic capacitor

from EN to GND.

An internal zener diode on the EN pin clamps the

EN pin voltage to prevent running away. The

maximum pull up current (assuming the worst

case 6V for the internal zener clamp) should be

limited to 1mA or less.

Therefore, when driving EN with an external logic

signal, the driving voltage should be less than 6V.

When connecting EN to IN through a pull-up

resistor or a resistive voltage divider, select a

resistance that ensures a maximum pull-up

current of 1mA.

exceeds 6V, then the minimum resistance for the

IN

UP

DOWN

V6V

6V

1mA

RR





(6)

not connected), then the VCC UVLO threshold

value is:

IN

UP

V6V

R( )

1mA





(7)

A typical pull-up resistor is 100kΩ.

VCC Power Supply

The MPM3686 has an internal VCC LDO to

supply the power to the internal circuits and

drives the power devices. This VCC LDO is

derived from the input supply. To ensure proper

operation, the minimum input voltage should be

4.5V.

An external 5V VCC bias can disable the internal

LDO. In this case, Vin can be as low as 2.5V.

The efficiency can be higher with external 5V

VCC bias. Figure 6 shows the comparison.