FSFR1700HSL FAIRCHILD [Fairchild Semiconductor], FSFR1700HSL Datasheet - Page 9

• Manufacturer Part Number: FSFR1700HSL
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2011 Fairchild Semiconductor Corporation
FSFR1800 / FSFR1700-HS • Rev.1.0.0
Functional Description
1. Basic Operation: FSFR-HS series is designed to drive
high-side and low-side MOSFETs complementarily with
50% duty cycle. A fixed dead time of 350ns is introduced
between consecutive transitions, as shown in Figure 18.
Once LV
starts to operate, generates the low-side gate signal, and
drives the low-side MOSFET. The bootstrap diode and
capacitor is charged by the low-side MOSFET’s
operation. After the voltage on HV
HV
generated for the MOSFET.
2. Internal Oscillator: FSFR-HS series employs a
current-controlled oscillator, as shown in Figure 19.
Internally, the voltage of the RT pin is regulated at 2V
and the charging / discharging current for the oscillator
capacitor, C
out of the RT pin (I
the switching frequency increases as I
3. Frequency Setting: Figure 20 shows the typical
voltage gain curve of a resonant converter, where the
gain is inversely proportional to the switching frequency
in the ZVS region. The output voltage can be regulated
by modulating the switching frequency. Figure 21 shows
the typical circuit configuration for the RT pin, where the
opto-coupler transistor is connected to the RT pin to
modulate
frequency may be controlled from 20kHz to 500kHz.
The minimum switching frequency is determined as:
f
min
CC,START

792
Figure 19. Current-Controlled Oscillator
Figure 18. MOSFET Gate Drive Signals
CC
p
, typically 9.2V, the high-side gate signal is

the
T
R
is higher than LV
, is obtained by copying the current flowing
min
1

switching
0
.
CTC
54
µ
) using a current mirror. Therefore,
[
Hz
]
frequency.
CC,START
CC
CTC
= 12.5V, the IC
increases up to
increases.
The
switching
(1)
9
Assuming
transistor is 0.2V, the maximum switching frequency is
determined as:
To prevent excessive inrush current and overshoot of
output voltage during startup, the IC needs to increase
the voltage gain of the resonant converter progressively.
Since the voltage gain of the resonant converter is
inversely proportional to the switching frequency, soft-
start is implemented by sweeping down the switching
frequency from an initial high frequency (f
output voltage is established.
The soft-start circuit is constructed by connecting R-C
series network to the RT pin, as shown in Figure 21.
Initially, the operating frequency is set by the parallel
impedance of R
f
max
Figure 20. Resonant Converter Typical Gain Curve

792
Figure 21. Frequency Control Circuit
p

the
R
min
SS
||
1
R
saturation
and R
max

0
min
.
54
.
µ
[
voltage
Hz
]
of
www.fairchildsemi.com
I S S
opto-coupler
) until the
(2)