MP8736– HIGHEFFICIENCY,FAST TRANSIENT,6A, 19V SYNCHRONOUSBUCKCONVERTERIN ATINY3X4mmPACKAGE

MP8736 Rev. 1.34

www.MonolithicPower.com

13

4/18/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP8736 is a fully integrated synchronous

rectified step-down switch mode converter.

Constant-on-time (COT) control is employed to

provide fast transient response and easy loop

stabilization. At the beginning of each cycle, the

high-side MOSFET (HS-FET) is turned ON when

voltage. The ON period is determined by the

input voltage and the frequency-set resistor as

follows:

()

( )

()

()

ON

IN

6R7 k

Tns

40 ns

VV

0.4

×Ω

=+

−

(1)

After the ON period elapses, the HS-FET is

turned off, or becomes OFF state. It is turned ON

operation this way, the converter regulates the

output voltage. The integrated low-side MOSFET

(LS-FET) is turned on when the HS-FET is in its

OFF state to minimize the conduction loss. There

will be a dead short between input and GND if

both HS-FET and LS-FET are turned on at the

same time. It’s called shoot-through. In order to

avoid shoot-through, a dead-time (DT) is

internally generated between HS-FET off and LS-

FET on, or LS-FET off and HS-FET on.

Heavy-Load Operation

Figure 2—Heavy Load Operation

As Figure 2 shows, when the output current is

high, the HS-FET and LS-FET repeat on/off as

described above. In this operation, the inductor

current will never go to zero. It’s called

continuous-conduction-mode (CCM) operation. In

fairly constant.

Light-Load Operation

At light load or no load condition, the output

drops very slowly and the MP8736 reduces the

switching frequency automatically to maintain

high efficiency. The light load operation is shown

the inductor current is 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 to less than -1mA.

Hence, the output capacitors discharge slowly to

GND through LS-FET. As a result, the efficiency

at light load condition is greatly improved. At light

load condition, the HS-FET is not turned ON as

frequently as at heavy load condition. This is

called skip mode.

Figure 3—Light Load Operation

As the output current increases from the light

load condition, the time period within which the

current modulator regulates becomes shorter.

The HS-FET is turned ON more frequently.

Hence,

the

switching

frequency

increases

correspondingly. The output current reaches the

critical level when the current modulator time is

zero. The critical level of the output current is

determined as follows:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V

−

×

=

××

×

(2)