RT9595 查看數據表（PDF） - Richtek Technology

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RT9595

MOSFET Integrated Smart Photoflash Capacitor Charger with IGBT Driver

Richtek Technology 

RT9595

Application Information

The RT9595 integrates a constant peak current controller

for charging photoflash capacitor and an IGBT driver for

igniting flash tube. The photoflash capacitor charger uses

constant primary peak current and SW falling control to

efficiently charge the photoflash capacitor.

Pulling the CHARGE pin high will initiate the charging

cycle. However, the CHARGE signal must come from low

to high after VDD > 2V for at lease 1μs delay time.

VDD

2V

CHARGE

>1µs

Figure 1. Recommend Charge Timing Chart

During MOSFET on period, the primary current ramps up

linearly according to VBAT and primary inductance. A resistor

connecting from CS pin to GND determines the primary

peak current.

During the MOSFET off period, the energy stored in the

flyback transformer is boosted to the output capacitor. The

secondary current decreases linearly at a rate determined

by the secondary inductance and the output voltage

(neglecting the voltage drop of the diode).

The SW pin monitors the secondary current. When the

secondary current drops to 0A, SW voltage falls, and then

the MOSFET on period starts again. The charging cycle

repeats itself and charges the output capacitor. The output

voltage is sensed by a voltage divider connecting to the

anode of the rectifying diode. When the output voltage

reaches the desired voltage set by the resistor divider,

the charging block will be disabled and charging will be

stopped.

Then STAT pin will be pulled low to indicate the complete

charging.

The voltage-sensing path will be cut off when charging is

completed to minimize the output voltage decay. Both

the CHARGE and STAT pins can be easily interfaced to a

microprocessor in a digital system.

Copyright ©2012 Richtek Technology Corporation. All rights reserved.

www.richtek.com

6

Charge Current Setting

The RT9595 simply adjusts peak primary current by a

resistor RCS connecting to the CS pin as shown in the

Function Block Diagram. RCS determines the peak current

of the primary N-MOSFET according to the following

equation :

IPK_PRI

=

40000

RCS

(A)

Where the IPK-PRI is the primary peak current. Users could

select appropriate RCS according to the battery capability

and required charging time.

Transformer

The flyback transformer should be appropriately designed

to ensure effective and efficient operation.

1. Turns Ratio

The turns ratio of transformer (N) should be high enough

so that the absolute maximum voltage rating for the

internal N-MOSFET drain to source voltage is not

exceeded. Choose the minimum turns ratio according to

the following formula :

NMIN

≥

VOUT

40 − VBAT

Where :

VOUT : Target Output Voltage

VBAT : Battery Voltage

2. Primary Inductance

Each switching cycle, energy transferred to the output

capacitor is proportional to the primary inductance for a

constant primary current. The higher the primary

inductance, the higher the charging efficiency. Besides,

to ensure enough off time for the output voltage sensing,

the primary inductance should be high enough according

to the following formula :

LPRI

≥

400 ×10-9 × VOUT

N × IPK-PRI

VOUT : Target Output Voltage

N : Transformer turns ratio

IPK-PRI : Primary peak current

is a registered trademark of Richtek Technology Corporation.

DS9595-04 March 2012

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