MP2148 Rev. 1.11

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5/25/2015

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on-timer generator, the operation mode is

below in Figure 6.

HS_ driver

VREF

VFB

COT

Generator

EN

IAAM

IL_ sense

R

SQ

FBCOMP

AAMCOMP

Figure 5: Simplified AAM Control Logic

Figure 6: AAM Comparator Control Ton

If the AAM comparator pulse is shorter than on-

timer generator, the operation mode is below in

Figure

7.

Generally,

using

very

small

inductance may bring this case.

AAM

Io

IL

b. Increase load in on-timer

AAM

Io

IL

a. Light load in on-timer

Figure 7: On-Timer Control Ton

Except upper on-timer method, AAM circuit has

another 150ns AAM blank time in sleep mode.

That means if on-timer is less than 150ns, the

high side MOSFET may turn off after on-timer

generator pulse without AAM control. Just a

remind, the on-time pulse at sleep mode is

around 40% larger than that on DCM or CCM

threshold as Figure 8.

Figure 8: AAM Blank Time in Sleep Mode

Figure9 shows AAM threshold decreases with

Ton increasing gradually. For the CCM state, Io

needs more than half of AAM threshold at least.

Figure 9: AAM Threshold decreases with Ton

Increases.

MP2148 have a zero current cross detect circuit

(ZCD) to judge if the inductor current starts to

reverse. When the inductor current reaches

ZCD threshold, the low side switch will be

turned off.

AAM mode together with ZCD circuit make

MP2148 always work on DCM mode at light

load, even if Vo is closed to Vin.

Enable

When input voltage is greater than the under-

voltage lockout threshold (UVLO), typically 2V,

MP2148 can be enabled by pulling EN pin to

higher than 1.2V. Leaving EN pin float or pull

down to ground will disable MP2148. There is

an internal 1Meg Ohm resistor from EN pin to

ground.

When the device is disabled, the part goes into

output discharge mode automatically and its

internal discharge MOSFET provides a resistive

discharge path for the output capacitor.

Soft Start

The MP2148 has a built-in soft start that ramps

up the output voltage at a controlled slew rate

to avoid overshoot at startup. The soft start time

is about 0.5ms typically.