MP1492 – 2A, 4.2V-16V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER

MP1492 Rev. 1.1

www.MonolithicPower.com

9

3/13/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP1492 is a fully integrated synchronous

rectified step-down switch converter. Adaptive

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

the feedback voltage (FB) is below the reference

voltage (REF) which indicates insufficient output

voltage. The ON period is determined by the

input voltage and the frequency-set resistor as

follows:

×Ω

=+

−

7

ON

IN

9.3 R (k )

T(ns)

40ns

V(V) 0.4

(1)

After the ON period elapses, the HS-FET is

turned off. It is turned ON again when FB drops

below REF. By repeating operation in 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.

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

CCM

operation,

the

switching frequency (Fs) is fairly constant.

Light-Load Operation

When the load current decreases, MP1492

reduces the switching frequency automatically to

maintain high efficiency. As the output current

reduces from heavy-load condition, the inductor

current decreases as well, and eventually comes

close to zero current. The LS-FET driver turns

into tri-state (high Z) whenever the inductor

current reaches zero level. The current modulator

takes over the control of LS-FET and limits the

inductor current to less than -1mA.

Hence,

efficiency at light-load condition is optimized.

Figure 2

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. 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

charged from VCC through N1 (Figure 3). N1

turns on when LS switches turns on and turns off

when LS switch turns off.

Switching Frequency

Adaptive constant-on-time (COT) control is used

in MP1492 and there is no dedicated oscillator in

the IC. The input voltage is feed-forwarded to the

switching frequency is fairly constant over the

input voltage range.

The switching frequency

can be set as follows:

=

×Ω

×+

−

6

S

7IN

DELAY

IN

OUT

10

F(kHz)

9.3 R (k )

V (V)

T(ns)

V(V) 0.4

V

(V)

(2)

Where TDELAY is the comparator delay, it’s

about 40ns.

MP1492 is optimized to operate at high switching

frequency but with high efficiency. High switching

frequency makes it possible to utilize small sized

LC filter components to save system PCB space.

Jitter and FB Ramp Slope

Figure 3 and Figure 4 show jitter occurring in

both PWM mode and skip mode. When there is