MP3910 datasheet(17/23 Pages) MPS

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MP3910 –PEAK CURRENT MODE BOOST CONTROLLER

MP3910 Rev. 1.11

www.MonolithicPower.com

11/6/2014

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OUT

V(V

V )





 

OUT

LOAD







I(30% 50%) I







Where ILOAD is the load current, ΔI is the peak-

to-peak inductor ripple current and η is the

efficiency. For a typical design, boost converter

efficiency can reach 85%~95%.

The switch current is usually used for the peak

current mode control. In order to avoid hitting

the current limit, the voltage across the sensing

resistor RSENSE should be less than 80% of the

worst case current limit voltage, 185mV.

SENSE

L(PEAK )

0.8 0.185





Where IL(PEAK) is the peak value of the inductor

current.

Selecting the Power MOSFET

The MP3910 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selecting a MOSFET are:

1. Maximum drain to source voltage, VDS(MAX)

2. Maximum current, ID(MAX)

3. On-resistance, RDS(ON)

4. Gate source charge QGS and gate drain

charge QGD

5. Total gate charge, QG

6. Turn on threshold VTH

Ideally, the off-state voltage across the

MOSFET is equal to boost output voltage.

Considering the voltage spike when it turns off,

VDS(MAX) should be greater than 1.5 times of the

output voltage.

The maximum current through the power

MOSFET happens when the input voltage is

minimum and the output power is maximum.

The maximum RMS current through the

MOSFET is given by:

OUT

RMS

OUT





The current rating of the MOSFET should be

greater than 1.5 times IRMS,

The on resistance of the MOSFET determines

the conduction loss, which is given by:

LOSS

RMS

DS(ON)





Where K is the on-resistance temperature

coefficient of the MOSFET. So it is smaller, it is

better.

The switching loss is related to QGD and QGS1

which determine the commutation time. QGS1 is

the charge between the threshold voltage and

the plateau voltage when a driver charges the

gate, which can be read in the chart of VGS vs.

QG of the MOSFET datasheet. QGD is the charge

during

the

plateau

voltage.

These

two

parameters are needed to estimate the turn on

and turn off loss.

GS1

PLT

















Where VTH is the threshold voltage, VPLT is the

plateau voltage, RG is the gate resistance, VDS

is the drain-source voltage. Please note that the

switching loss is the most difficult part in the

loss estimation. The formula above provides a

simple physical expression.

On the other hand, small QG will cause fast turn

on/off speed which determines the spike and

kick.

The turn on threshold voltage VTH is also

important. GATE pin is powered by VCC power,

so the VTH must be lower than VCC voltage. For

worst case, VCC voltage is at the 3.55V falling

UVLO, so the MOSFET VTH should be much

lower than this voltage to get enough driving

capability.

Selecting the Diode

The boost output rectifier diode supplies current

to the inductor when the MOSFET is off. Use a

Schottky diode to reduce losses due to the

diode forward voltage and recovery time. The

diode should be rated for a reverse voltage

greater than the expected output voltage. The

average current rating must exceed the

maximum expected load current, and the peak

current rating must exceed the peak inductor

current.

Boost Converter Compensation Design

The output of the transconductance error

amplifier (COMP) is used to compensate the

regulation control system. The system uses two

poles and one zero to stabilize the control loop.

The poles are FP1, which is set by the output

capacitor (COUT) and load resistance, and FP2

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