MP3351 datasheet(7/8 Pages) MPS

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MP3351 – 60V, 2A INTEGRATED PHOTO FLASH CHARGER WITH IGBT DRIVER

MP3351 Rev. 0.91

www.MonolithicPower.com

9/2/2008

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

APPLICATION INFORMATION

MP3351 controlled flyback charger operates in

critical conduction mode with peak current set

by ISET pin. Output voltage is divided down

through R1, R2 and R3 from the positive

terminal of the transformer secondary and

compares with 1.20V at FB pin. The low to high

transition of CHARGE pin will enable the

flyback converter to switch.

A minimum TOFF of 20µS provides a minimum

OFF time at the beginning of charging when

DCM comparator always trips because VOUT is

still near zero, Min TOFF will be reduced to

400nS after VO>16V. Charger may enter DCM

mode when charge is close to full and actual

OFF time is less than minimum TOFF. Circuit

will stop switching and RDYB will be pulled low

once the flash capacitor is charged to 300V, a

value set by R1, R2 and R3. When the part is

ready the internal dissipation is reduced to just

the circuits for IGBT driver. Toggle CHARGE

pin will start charge again. Bringing CHARGE

pin low terminates the power delivery and put

the part in shutdown. A maximum TON timer

prevents pulling current from starved power

source. If the ON time exceeds maximum TON,

the switch is forced OFF regardless of IPEAK

detection. Integrated IGBT driver uses VIN as its

power supply.

DCM Comparator

DCM boundary is reached when V(FB) drops to

zero. The DCM comparator compares V(FB)

and a fixed offset voltage of 25mV to account

for the comparator delay.

Charge Speed

The output capacitor charging speed is

determined by:

LIM

CHARGE

OUT

∝

Setting Peak Charge Current

MP3351’s peak charge current (ISET) can be set

by an external resistor, RSET from the ISET pin

to ground. The value of RSET can be calculated

from:

SET

R(K )

I(A)

Ω=

For example, for 2A peak charge current, RSET

is 24.9kΩ.

Primary Inductance

The primary inductance is calculated based on

the minimum off time period:

OUT MIN,OFF

PRIMARY

PEAK

≥

VOUT: output voltage about 300V

TMIN, OFF: Minimum off time 400nsec.

IPEAK: primary peak current

Turns Ratio

The minimum turns ratio of the flyback

transformer is obtained as:

OUT

≥

−

VDS: FET drain-source voltage

VIN: Input voltage 3~5V

Setting Output Voltage

The output voltage is set by selecting the

resistive voltage divider ratio. If we use

1.2kFor the low side resistor (R3) of the

voltage divider, we can determine the high side

resistor (R1, R2) by the equation:

OUT

R1 R2

−

Typical

values

are

R1=R2=150kΩ

and

R3=1.2kΩ.

Output Diode Selection

The reverse voltage of the output diode is

determined by:

DOUT

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