MP38900/MP38900-B–HIGHEFFICIENCY,FASTTRANSIENTSYNCHRONOUSSTEP-DOWNCONVERTER

MP38900/MP38900-B Rev. 1.1

www.MonolithicPower.com

12

6/25/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP38900/MP38900-B 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

reference

voltage

which

indicates

insufficient output voltage. The ON period is

determined by the input voltage and the

frequency-set resistor as follows:



(ns)

t

0.45

(V)

V

k

R7

12

ns

t

DELAY1

IN

on











)

(

(1)

Where R7 is the resistor to set switching

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 3: Heavy Load Operation

As Figure 3 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

When

the

load

current

decreases,

The

MP38900/MP38900-B reduces the switching

frequency

automatically

to

maintain

high

efficiency. The light load operation is shown in

inductor current is approaching zero. As the

output current reduces from heavy-load condition,

the inductor current also decreases, and

eventually comes close to zero. The LS-FET

driver turns into tri-state (high Z) whenever the

inductor current reaches zero level. A current

modulator takes over the control of LS-FET and

limits the inductor current to less than 600μA.

Hence, the output capacitors discharge slowly to

GND through LS-FET as well as R1 and R2. 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 4: 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: