RT9595 데이터 시트보기 (PDF) - Richtek Technology
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RT9595 Datasheet PDF : 9 Pages
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RT9595
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
by N2 when reflected to the primary will become large.
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 :
VPK-R ≈ VOUT + (N × VBAT )
The peak current of the diode equals the primary peak
current divided by the transformer turn ratio as the following
equation :
IPK-SEC
=
IPK-PRI
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.
If VOUT = 300V, according the following equation :
VOUT
=
VFB
×
(1 +
R1+R2 )
R3
and
R1+R2
R3
= 299
It is recommended to set R3 = 1kΩ 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.
VOUT
R1
150k
R2
150k
FB
R3
1k
COUT
Figure 2. Sensing Anode of Diode
VOUT
R1
10M
R2
10M
COUT
FB
C1
10nF
R3
66.5k
Figure 3. Sensing Output Voltage
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 :
VBAT
< 10V
−
0.16 × (R1+R2+R3)
N × R3
N : Transformer turns ratio.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS9595-04 March 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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