FAN7554 Fairchild Semiconductor, FAN7554 Datasheet - Page 9

• Manufacturer Part Number: FAN7554
• Description: IC CTRLR PWM CURRENT MODE 8DIP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN7554

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 500kHz
• Duty Cycle: 98%
• Voltage - Supply: 9 V ~ 30 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -25°C ~ 85°C
• Package / Case: 8-DIP (0.300", 7.62mm)
• Frequency-max: 500kHz
• Output Voltage: 30 V
• Output Current: 1 A
• Switching Frequency: 500 KHz
• Mounting Style: Through Hole
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 25 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: FAN7554_NL; FAN7554_NL

Oscillator
As shown in figure14, the oscillator frequency is programmed by values selected for timing components Rt and C
C
The oscillator generates the clock signal while the timing capacitor C
the clock time. Rt and C
can be calculated through the equations below.
t
Ct
Ct
Rt
Rt
is charged to almost 2.8V through resistor Rt from the 5V reference and discharged to 1.2V by an internal current source.
Ct
CT
CT
Vref
Vref
4
4
8
8
Cs
Cs
Figure 14. Oscillator Circuit
Discharge
Discharge
Discharge
t
2.8V
2.8V
/1.2V
/1.2V
selection determine the oscillator frequency and maximum duty cycle. Charge and discharge times
+
-
Charging time : tc = 0.55 R
Discharging time : td = R
where the oscillator frequency : fosc = (tc + td)
When R
Cfb
Cfb
Figure 13. Soft Start Circuit & Circuit Flow
S/S
S/S
2
2
FB
FB
t
Clock
1
1
Gate Drive
Gate Drive
> 5k , fosc = 1 / (0.55 R
FAN7554
FAN7554
1mA
1mA
5uA
5uA
Ds
Ds
5V
5V
Sawtooth waveform
Sawtooth waveform
Sawtooth waveform
Sawtooth waveform
100uA
100uA
Vcc
Vcc
t
C
t
Vlow(1.2V)
Vlow(1.2V)
Vlow(1.2V)
Vlow(1.2V)
Vhigh(2.8V)
Vhigh(2.8V)
Internal clock
Internal clock
Vhigh(2.8V)
Vhigh(2.8V)
Internal clock
Internal clock
t
C
ln[(0.0063 R
t
t
is discharged. The gate drive output becomes low during
5V
5V
t
2R
2R
C
t
R
R
R
R
) = 1.8 / (R
Figure 15. Sawtooth & Clock Waveform
t
tc
tc
- 2.8) / (0.0063 R
tc
tc
-1
(±10%)
td
td
t
td
td
C
t
)
FAN7554
FAN7554
t
- 3.8)]
Output drive
Output drive
t
. Capacitor
[ Rt > 5k ]
[ Rt > 5k ]
[ Rt < 5k ]
[ Rt < 5k ]
FAN7554
9