MP8760 — 6A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8760 Rev. 1.1

www.MonolithicPower.com

12

11/4/2013

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OPERATION

PWM Operation

The MP8760 is a fully-integrated, synchronous,

rectified, step-down, switch-mode converter. It

uses constant-on-time (COT) control to provide a

fast

transient

response

and

ease

loop

stabilization.

At the beginning of each cycle, the high-side

MOSFET (HS-FET) turns ON when the feedback

voltage. The input voltage and the frequency-set

resistor determine the ON period as follows:

4

0

3

5

.

)

V

(

V

)

k

(

R

.

)

ns

(

IN

FREQ

ON

−

Ω

×

=

τ

(1)

After the ON period elapses, the HS-FET turns

By repeating this operation, the converter

regulates the output voltage. The integrated low-

side MOSFET (LS-FET) turns ON when the HS-

FET is OFF to minimize conduction loss and

avoid a dead short (or shoot-through) between

input and GND if both HS-FET and LS-FET turn

on at the same time. An internally-generated

dead-time (DT) between HS-FET OFF and LS-

FET ON or LS-FET OFF and HS-FET ON avoids

shoot-through.

Heavy-Load Operation

Figure 2: Heavy-Load Operation

When the output current is high and the inductor

current is always above zero amps, it is called

continuous-conduction-mode (CCM). Figure 2

FET turns ON for a 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 next period.

In CCM, the switching frequency is fairly constant

and it is also called PWM mode.

Light-Load Operation

As the load decreases, the inductor current

decreases. The operation transitions from CCM

to discontinuous-conduction-mode (DCM) when

the inductor current reaches 0A.

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

Instead, the LS-FET driver enters tri-state (high

Z). A current modulator then controls the LS-FET

and limits the inductor current to less than −1mA.

Hence, the output capacitors discharge slowly to

GND through LS-FET, and the HS-FET doesn’t

turn ON as frequently as under heavy-load

conditions, thus greatly improving light-load and

no-load efficiency. This is called skip mode.

Figure 3: Light-Load Operation

As the output current increases from the light-

load,

the

current

modulator

shortens

the

operating period to turn the HS-FET ON more

frequently. Hence, the switching frequency

increases. The output current reaches its critical

threshold when the current modulator time

decreases to zero. Determine the critical output

current level as follows:

IN

SW

OUT

OUT

IN

OUT

V

f

L

V

)

V

V

(

I

×

×

×

×

−

=

2

(2)

The IC enters PWM mode once the output

current exceeds its critical level. Then the