MPM3686 – 18V 20A STEP-DOWN POWER MODULE

MPM3686 Rev. 1.02

www.MonolithicPower.com

12

10/13/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

Light-Load Operation

As the load decreases, the inductor current

decreases too. When the inductor current

touches zero, the operation is transited from

continuous-conduction-mode

(CCM)

to

discontinuous-conduction-mode (DCM).

Figure 3 shows the light load operation. When

fixed interval determined by the one- shot on-

timer as per equation 1. When the HS-FET turns

off, the LS-FET turns on until the inductor current

approaching zero. The LS-FET driver turns into

tri-state (high Z) whenever the inductor current

reaches zero. A current modulator takes over the

control of LS-FET and limits the inductor current

less than -1mA. Hence, the output capacitors

discharge slowly to GND through LS-FET. As a

result, this mode improves the efficiency greatly

at light load condition. At this condition, the HS-

FET does not turns ON as frequently as at heavy

load condition. This is called pulse skip mode.

At light load or no load condition, the output

drops very slowly and the MPM3686 reduces the

switching frequency naturally and then achieves

high efficiency at light load.

Figure 3—Light Load Operation

As the output current increases from the light

load condition, the current modulator regulates

the operating period that becomes shorter. The

HS-FET turns ON more frequently. Hence, the

switching frequency increases correspondingly.

The output current reaches the critical level when

the current modulator time decreases to zero.

The

critical

output current

level

can

be

determined as follows:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V









(2)

The IC turns into PWM mode once the output

current exceeds the critical level. After that, the

switching frequency stays fairly constant over the

output current range.

Switching Frequency

Selecting the switching frequency requires

trading off between efficiency and component

size.

Low

frequency

operation

increases

efficiency by reducing MOSFET switching losses,

but requires larger inductor and capacitor values

to minimize the output voltage ripple.

The MPM3686 uses adaptive constant-on-time

(COT) control to generate a fairly constant

frequency at CCM condition, though the IC lacks

a dedicated oscillator. The ON time of HSFET

can be set by connecting a resistor between IN

pin and FREQ pin. It’s input voltage adaptive. So

for a fixed output voltage, the switching

frequency stays fairly constant. The switching

frequency can be set internally and externally.

Figure 4 shows that the switching frequency is

determined by the internal 430K resistor. The

430K resistor is connected to IN pin so that the

input voltage is feed-forwarded to the one-shot

ON-time timer. When operating in steady state at

switching frequency is fairly constant over the

input voltage range. The switching frequency can

be determined by equation 3:

Figure 4

6

SW

IN

DELAY

IN

OUT

10

F(kHz)

V(V)

6.1 430(k )

T(ns)

V(V) 0.4

V

(V)









(3)

5ns.