MP2321 Rev. 1.0

www.MonolithicPower.com

13

10/26/2016

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The maximum frequency can be calculated by

Equation (4) and Equation (5). Choose the

lower value of 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)

Where the Ton-min typical value is 90ns, and

the Toff-min typical value is 150ns. For example,

maximum frequency is about 1.1MHz. The

MP2321 is optimized to operate at a high

switching frequency with high efficiency. High

switching frequency allows small LC filter

components to be used to save system PCB

space.

Forced PWM Operation

When the MP2321 works in forced PWM mode,

the MP2321 enters CCM, where the HS-FET

and LS-FET repeat the on/off operation, even if

the inductor current goes to zero or a negative

constant. Figure 4 shows the timing diagram

during this operation.

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

Light-Load Operation

When the MP2321 works in auto PFM/PWM

mode and during light-load operation, the

MP2321 automatically reduces the switching

frequency to maintain high efficiency, and the

inductor current drops almost to zero. When the

inductor current reaches zero, the LS-FET

driver goes into tri-state (high-Z). Therefore, the

output capacitors discharge slowly to GND

through LS-FET, R1, and R2. This operation

greatly improves device efficiency when the

output current is low (see Figure 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 in heavy-load condition.

The frequency at which the HS-FET turns on is

a function of the output current. As the output

current increases, the time period that the

current modulator regulates becomes shorter,

and the HS-FET turns on more frequently. The

switching frequency increases in turn. The output

current reaches critical levels when the current

modulator time is zero, and can be determined

with Equation (6):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V





 

(6)

The device resumes PWM mode once the

output current exceeds the critical level.

Afterward, the switching frequency remains

fairly constant over the output current range.

High Duty Cycle Condition

The MP2316 extends the on time when the

output voltage loses regulation when the input

voltage is close to the output voltage. The

switching frequency drops correspondingly to

achieve a larger duty cycle to keep the output

regulated.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN through

D1, M1, Cb, L1, and C2A (see Figure 6). If VIN

a 5V BST voltage across Cb.