RT9591 查看數據表（PDF） - Richtek Technology

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RT9591

Smart Photoflash Capacitor Charger with IGBT Driver

Richtek Technology 

Preliminary

RT9591

Application Information

The RT9591 integrates a constant peak current controller

for charging photoflash capacitor, an IGBT driver for

igniting flash tube, and a voltage detector with open drain

output to provide a cost effective photoflash solution.

The photoflash capacitor charger uses constant primary

peak current and constant secondary valley current control

to efficiently charge the photoflash capacitor. Pulling the

CHARGE pin high initiates the charging cycle. During ON

time, the primary current ramps up linearly according to

VBAT and primary inductance. A resistor connecting to CS

pin determines the ON time of primary NMOS and

consequently the primary peak current.

During the OFF time, 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 secondary

current is monitored by the IMCD pin. When the secondary

current drops below 10mA, ON time starts again. The

charging cycle repeats itself and charges the output

voltage.

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

resistor divider, the HV detector will terminate the charging

cycle, disable the charging block and pull high the STAT

pin. The voltage sensing path is cut off when charging

completed to minimize the output voltage decay. Both

the CHARGE and STAT pins can be easily interfaced to a

microprocessor in a digital system.

Transformer

The flyback transformer should be appropriately designed

to ensure effective and efficient operation.

VOUT: Target Output Voltage

VDS(MAX): Maximum drain to source voltage of NMOS

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 is, the higher the charging efficiency will be.

Besides, the RT9591 has a 360ns minimum-off time for

correct current and voltage sensing. To ensure the charger

operating in continuous conduction mode, the primary

inductance should be high enough according to the

following formula:

LPRI

≥

430 x 10-9 VOUT

N x IPK - PRI

VOUT: Target Output Voltage

N : Transformer turns ratio

IPK-PRI : Primary peak current

430 x 10−9 : The maximum value of minimum-off time.

3. Leakage Inductance and Parasitic Capacitance

The leakage inductance of the transformer results in the

first spike voltage when NMOS turns off as shown in

Figure 4. The spike voltage is proportional to the leakage

inductance. The spike voltage must not exceed the

dynamic rating of the NMOS drain to source voltage. Well-

coupling winding design decreases the leakage

inductance. However, well-coupling winding design

usually results in large parasitic capacitance between

windings. The parasitic capacitance consequently causes

initial current swing when NMOS turns on as shown in

Figure 5. Trade off is necessary between leakage

inductance and parasitic capacitance.

1. Turns Ratio

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

so that the absolute maximum voltage rating for the NMOS

drain to source voltage is not exceeded. Choose the

minimum turns ratio according to the following formula:

N(MIN) ≥

VOUT

VDS(MAX) - VBAT

DS9591-07 August 2007

Patent Pending

www.richtek.com

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