RT9595

7

DS9595-04

March 2012

www.richtek.com

©

Copyright

2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

PK-R

OUT

BAT

VV

(N V

)

≈+

×

The peak current of the diode equals the primary peak

current divided by the transformer turn ratio as the following

equation :

PK-PRI

PK-SEC

I

I

N

=

Where : N is the transformer turns ratio.

Output Voltage Setting

The RT9595 senses the output voltage by a voltage divider

connecting to the anode of the rectifying diode during OFF

time as shown in Figure 2. This eliminates power loss at

voltage-sensing circuit when charging is completed. R1

to R2 ratio determines the output voltage as shown in the

typical application circuit. The feedback reference voltage

is 1V.

OUT

FB

R1+R2

R1+R2

V

V

(1

) and

299

R3

R3

=× +

=

It is recommended to set R3 = 1k

Ω and R1 = R2 = 150kΩ

for reducing parasitic capacitance coupling effect of the

FB pin. R1 and R2 MUST be greater than 0805 size

resistor for enduring secondary HV. Another sensing

method is to sense the output voltage directly as shown

in Figure 3.

Figure 2. Sensing Anode of Diode

Figure 3. Sensing Output Voltage

R3

R1

1k

R2

150k

150k

FB

R3

R1

66.5k

R2

10M

10M

FB

C1

10nF

Over Voltage Protection (OVP)

The RT9595 provides over voltage protection (OVP)

function. In the typical application circuit, if the FB resistor

R1, R2 or R3 is open, the FB voltage will be pulled low or

floating. In this condition, when the CHARGE pin goes

High, the RT9595 begins switching, once the SW voltage

rises to higher than 10V, the OVP function will be triggered.

The avoiding OVP battery voltage upper limit is shown as

the following equation :

3. Leakage Inductance

The leakage inductance of the transformer results in the

first spike voltage when N-MOSFET turns off. The spike

voltage is proportional to the leakage inductance and

inductor peak current. The spike voltage must not exceed

the dynamic rating of the N-MOSFET drain to source

voltage (45V).

4. Transformer Secondary Capacitance

Any capacitance on the secondary can severely affect

the efficiency. A small secondary capacitance is multiplied

This capacitance forms a resonant circuit with the primary

leakage inductance of the transformer. Therefore, both the

primary leakage inductance and secondary side

capacitance should be minimized.

Rectifying Diode

The rectifying diode should be with short reverse recovery

time (small parasitic capacitance). Large parasitic

capacitance increases switching loss and lowers charging

efficiency.

In addition, the peak reverse voltage and peak current of

the diode should be sufficient.

The peak reverse voltage of the diode can be calculated

as following Equation :

BAT

0.16 (R1+R2+R3)

V10V

NR3

×

<−

×

N : Transformer turns ratio.