MPM3820 – 2A, 6V, SIMPLE MODULE WITH INTEGRATED INDUCTOR

MPM3820 Rev. 1.21

www.MonolithicPower.com

12

6/11/2015

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© 2015 MPS. All Rights Reserved.

OPERATION

The DC-DC module has a small surface-mount

QFN-20(3mm x 5mm x 1.6mm) package. The

module’s integrated inductor simplifies the

schematic and layout design. Only FB resistors

and input and output capacitors are needed to

complete the design. MPM3820 uses constant

on-time control (COT) with input voltage feed

forward to stabilize the switching frequency

over a full-input range. At light load, MPM3820

employs a proprietary control of the low-side

switch and inductor current on switching node

and improve efficiency.

Constant On-Time Control (COT)

Compared to a fixed-frequency PWM control,

constant on-time control (COT) offers the

advantage of a simpler control loop and faster

transient response. Using input voltage feed

forward, the MPM3820 maintains a nearly

constant switching frequency across the input and

output voltage range.

The on-time of the

switching pulse is estimated as follows:

OUT

ON

IN

V

T0.833 s

V

=×

μ

To prevent inductor current run away during load

transition, MPM3820 fixes the minimum off time

to 30ns. However, this minimum off-time limit

does not affect operation in a steady state.

Light Load Operation

In a light-load condition, MPM3820 uses a

proprietary control scheme to save power and

improve efficiency. There is a zero current

cross detect circuit (ZCD) to judge if the

inductor current starts to reverse. When the

inductor current touch ZCD threshold, the low

side switch will start to be turned off.

The DCM mode happens only after low side

switch turned off by ZCD circuit. Considering

the ZCD circuit propagation time, the typical

delay is 20ns. This means the inductor current

continues to fall after the ZCD is triggered. If the

inductor current falling slew rate is fast (Vo

voltage is high or close to Vin), the low-side

MOSFET turns off (this means the inductor

current may be negative). This does not allow

the MPM3820 to enter DCM.

If DCM is required, the off-time of the low-side

MOSFET in continuous conduction mode (CCM)

should be longer than 40ns. It means the

maximum duty is 95% to guarantee DCM mode

at light load.

For example, if Vin is 3.4V and Vo is 3.3V, the

off-time in CCM is 25ns. It is difficult to enter

DCM at light load.

Enable (EN)

If the input voltage is greater than the under-

voltage lockout threshold (UVLO), typically 2.5V,

MPM3820 is enabled by pulling EN above 1.2V.

Leaving EN to float or be pulled down to ground

disables MPM3820. There is an internal 1MΩ

resistor from EN to ground.

Soft Start/Stop

MPM3820 has a built-in soft-start that ramps up

the output voltage in a controlled slew rate. This

avoids overshoot at startup. The soft-start time

is about 1.5ms typically.

At disable, MPM3820 ramps down the internal

reference thus allow the load to linearly

discharge the output. During soft stop time, the

low side internal MOSFET will switch to control

the slew rate of output voltage which follows the

internal reference. Under light load and large

output capacitor condition, the large energy

stored in output capacitor will be transferred to

input capacitor through the inductor. The

topology is changed into a boost converter after

VOUT and VIN role exchange. The boost

voltage causes an overshoot on input capacitor;

sometimes this overshoot is higher than the

VABS (the ABSOLUTE maximum value) of

input pin and can damage the IC. To prevent

this situation, the input capacitor needs to be

large enough to absorb this energy.

The energy stored in the output capacitor will

be transferred to input capacitor. Consider the

conduction loss on inductor, HS/LS MOS and

so on, estimate 80% transfer efficiency of boost

converter. Therefore the transferred energy can

be calculated by below equation: