MP1499 – 16V/5A PEAK SYNCHRONOUS STEP-DOWN CONVERTER

MP1499 Rev. 1.0

www.MonolithicPower.com

11

7/26/2012

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© 2012 MPS. All Rights Reserved.

Figure 3—Adjustable UVLO

Soft-Start

The

MP1499

employs

soft

start

(SS)

mechanism to ensure smooth output during

power-up. When the EN pin becomes high, an

internal current source (11μA) charges up the

SS capacitor. The SS capacitor voltage takes

over the REF voltage to the PWM comparator.

The output voltage smoothly ramps up with the

SS voltage. Once the SS voltage reaches the

same level as the REF voltage, it keeps

comparator. At this point, the soft start finishes

and it enters into steady state operation.

If the output is pre-biased to a certain voltage

during startup, the IC will disable the switching

of both high-side and low-side switches until the

voltage on the internal soft-start capacitor

exceeds the sensed output voltage at the FB

pin.

The SS capacitor value can be determined as

follows:



 

 





SS

SS

REF

Tms

I

A

CnF

VV

(1)

If the output capacitors have large capacitance

value, it’s not recommended to set the SS time

too small. Otherwise, it’s easy to hit the current

limit during SS. A minimum value of 4.7nF

should be used if the output capacitance value

is larger than 330μF.

Power Save Mode for Light Load Condition

The

MP1499

has

AAM

(Advanced

Asynchronous Modulation) power save mode

for light load. The AAM voltage is set at 0.4V

internally. Under the heavy load condition, the

high, the high-side power MOSFET turns on

set by the COMP voltage. The internal clock

Under the light load condition, the value of

clock is blocked, thus the MP1499 skips some

pulses for PFM (Pulse Frequency Modulation)

mode and achieves the light load power save.

Clock

R1

1pF

50pF 400k

R2

HS_driver

+

-

QS

R

+

-

Figure 4—Simplified AAM Control Logic

When the load current is light, the inductor peak

current is set internally which is about 1A for

inductor peak current vs. inductor is shown in

Figure 5.

Inductor Peak Current

vs. Inductor

0

0.5

1

1.5

2

2.5

0123456

Figure5—Inductor Peak Current vs. Inductor

Over-Current-Protection and Hiccup

The MP1499 has cycle-by-cycle over current

limit when the inductor current peak value