FSB117H Fairchild Semiconductor, FSB117H Datasheet - Page 13

• Manufacturer Part Number: FSB117H
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSB117H / FSB127H / FSB147H • Rev. 1.0.4
Figure 23. Current Limit Variation with Duty Cycle
Adjustable Peak Current Limit & H/L Line
Compensation for Constant Power Limit
To make the limited output power constant regardless
of the line voltage condition, a special current-limit
profile with sample and hold is used (as shown in
Figure 22). The current-limit level is sampled and held
at the falling edge of gate drive signal as shown in
Figure 23. Then, the sampled current limit level is used
for the next switching cycle. The sample-and-hold
function prevents sub-harmonic oscillation in current-
mode control.
The current-limit level increases as the duty cycle
increases, which reduces the current limit as duty cycle
decreases. This allows lower current-limit level for high-
line voltage condition where the duty cycle is smaller
than that of low line. Therefore, the limited maximum
output power can remain constant even for a wide input
voltage range.
The peak current limit is programmable using a resistor
on the IPK pin. The internal current 50µA source for the
IPK pin generates voltage drop across the resistor. The
voltage of the IPK pin determines the current-limit level.
Since the upper and lower clamping voltage of the IPK
pin are 3V and 1.5V, respectively, the suggested
resistor value is from 30kΩ to 60kΩ.
Figure 21. Current Limit Variation During Soft-Start
Figure 22. I
LMT
vs. PWM Turn-On Time
13
mWSaver™ Technology
Ax-CAP ™ to Remove X-Cap Discharge Resistor
The EMI filter in the front end of the switched mode
power supply typically includes a capacitor across the
AC line connector, as shown in Figure 24. Most of the
safety regulations, such as UL 1950 and IEC61010-1,
require the capacitor be discharged to a safe level within
a given time after unplugged from the power outlet.
Typically a discharge resister across the capacitor is
used to ensure the capacitor is discharged naturally,
which however introduces power loss of the power
supply. As power level increases, the EMI filter capacitor
tends to increase, requiring a smaller discharge resistor
to maintain same discharge time. This typically results in
more power dissipation in high-power applications. The
innovative Ax-CAP™ technology intelligently discharges
the filter capacitor only when the power supply is
unplugged from the power outlet. Since the Ax-CAP™
discharge circuit is disabled in normal operation, the
power loss in the EMI filter size can be virtually removed.
Green Mode
The FSB-series modulates the PWM frequency as a
function of FB voltage, as shown in Figure 25. Since the
output power is proportional to the FB voltage in current-
mode control, the switching frequency decreases as load
decreases. In heavy-load conditions, the switching
frequency is 100kHz. Once V
(2.6V), the PWM frequency linearly decreases from
100kHz to 23kHz to reduce switching losses at light-load
condition. As V
frequency is fixed at 23kHz.
As V
Burst Mode operation, where PWM switching is
disabled. Then, the output voltage starts to drop, causing
the feedback voltage to rise. Once V
ZDCR
and disables switching, thereby reducing switching loss
to reduce power consumption, as shown in Figure 26.
, switching resumes. Burst Mode alternately enables
FB
falls below V
Figure 25.
Figure 24. AX-Cap™ Circuit
FB
decreases to V
FB-ZDC
PWM Frequency
(2.1V), the FSB-series enters
FB
FB-G
decreases below V
(2.4V), the switching
FB
rises above V
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FB-N
FB-