MP2326 Rev. 1.1

www.MonolithicPower.com

13

1/28/2016

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© 2016 MPS. All Rights Reserved.

Because of the minimum on time and minimum

off time, the switching frequency is limited. The

maximum frequency can be calculated by the

following equations. Choose the lowest value

between the two as the maximum frequency:

6

SW-max

IN

on-min

OUT

10

F(KHz)

V(V)

T(ns)

V(V)





(4)

6

IN

OUT

SW-max

off-min

IN

(V (V)-V (V)) 10

F(KHz)

T(ns) V (V)







(5)

that can be set is about 1.1MHz. The MP2326 is

optimized to operate at a high switching

frequency with high efficiency. A high switching

frequency makes it possible to use small-sized

LC filter components to save system PCB space.

Whenever V

below V

is turned ON

V

IN

T

V

RAMP

V

EAO

I

OUT

V

SW

HS-FET

Driver

LS-FET

Driver

I

L

FIGURE 4. Forced PWM Operation

When the MP2326 works in forced PWM, the

MP2326 enters CCM where the HS-FET and LS-

FET repeat the on/off operation (even if the

inductor current goes to zero or negative value).

Fig. 4 shows the timing diagram during forced

PWM operation.

Light-Load Operation

In auto PFM/PWM mode (during light-load

operation), the MP2326 reduces automatically

the switching frequency to maintain high

efficiency, and the inductor current drops near

zero. When the inductor current reaches zero,

the LS-FET driver goes into tri-state (high Z).

This causes the output capacitors to discharge

slowly to GND through the LS-FET, R1, and R2.

This operation greatly improves device efficiency

when the output current is low (see Fig. 5).

FIGURE 5. Light-Load Operation

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 frequency at which the HS-FET

turns on is a function of the output current. As the

output current increases, the time period in which

the current modulator regulates becomes shorter,

causing the HS-FET to turn on more frequently.

In turn, the switching frequency increases. The

output current reaches the critical level when the

current modulator time is zero. The output

current can be determined using the following

equation:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V





 

(6)

The device reverts to PWM mode once the

output current exceeds the critical level. Once in

PWM mode, the switching frequency stays fairly

constant over the output-current range.

Application when Input Voltage is Close to

Output Voltage

MP2326 extends the on time when the output

voltage looses regulation (when the input voltage

is close to the output voltage). The switching

frequency drops correspondingly in order to

achieve a larger duty cycle to keep the output

extension circuit forces MP2326 into PWM mode

with

a

higher

than

expected

frequency.

to exit PWM mode (see Fig. 6). The MP2326

above the curve. If auto PFM/PWM mode is

required at the input voltage below the curve, use

Enable start-up instead of the input voltage start-

up.