FAN302UL Datasheet, PDF (13/19 Pages) Fairchild Semiconductor – PWM Controller for Low Standby Power Battery-Charger Applications

I _HV
= ^{VDL −VHV}
R_HV
V_DL
R_HV
V_DL
Figure 34.V-I Characteristics of HV Pin
Frequency Hopping
EMI reduction is accomplished by frequency hopping,
which spreads the energy over a wider frequency range
than the bandwidth of the EMI test equipment, allowing
compliance with EMI limitations. The internal frequency-
hopping circuit changes the switching frequency
progressively between 135kHz and 145kHz with a
period of t_p, as shown in Figure 35.
Figure 36. Burst-Mode Operation
Slope Compensation
The sensed voltage across the current-sense resistor is
used for current-mode control and pulse-by-pulse
current limiting. A synchronized ramp signal with
positive slope is added to the current-sense information
at each switching cycle, improving noise immunity of
current-mode control.
Protections
The FAN302UL self-protection functions include V_DD
Over-Voltage Protection (V_DD OVP), internal Over-
Temperature Protection (OTP), V_S Over-Voltage
Protection (V_S OVP), and brownout protection. The V_DD
OVP and brownout protection are implemented as Auto-
Restart Mode, while the V_S OVP and internal OTP are
implemented as Latch Mode.
When an Auto Restart Mode protection is triggered,
switching is terminated and the MOSFET remains off,
causing V_DD to drop. When V_DD drops to the V_DD turn-off
voltage of 5V; the protection is reset, the internal startup
circuit is enabled, and the supply current drawn from the
HV pin charges the hold-up capacitor. When V_DD
reaches the turn-on voltage of 16V, normal operation
resumes. In this manner, auto restart alternately
enables and disables MOSFET switching until the
abnormal condition is eliminated, as shown in Figure 37.
Figure 35. Frequency Hopping
Burst-Mode Operation
The power supply enters Burst Mode at no-load or
extremely light-load conditionS. As shown in Figure 36,
when V_FB drops below V_FBL, the PWM output shuts off
and the output voltage drops at a rate dependent on
load current. This causes the feedback voltage to rise.
Once V_FB exceeds V_FBH, the internal circuit starts to
provide switching pulse. The feedback voltage then falls
and the process repeats. Burst Mode alternately
enables and disables switching of the MOSFET,
reducing the switching losses in Standby Mode. Once
FAN302UL enters Burst Mode, the operating current is
reduced from 3.5mA to 200μA to minimize power
consumption in Burst Mode.
© 2011 Fairchild Semiconductor Corporation
FAN302UL • Rev. 1.0.3
When a Latch Mode protection is triggered, PWM
switching is terminated and the MOSFET remains off,
causing V_DD to drop. When V_DD drops to the V_DD turn-off
voltage of 5V, the internal startup circuit is enabled
without resetting the protection, and the supply current
drawn from HV pin charges the hold-up capacitor. Since
the protection is not reset, the IC does not resume PWM
switching even when V_DD reaches the turn-on voltage of
16V, disabling the HV startup circuit. Then, V_DD drops
again down to 5V. In this manner, the Latch Mode
protection alternately charges and discharges V_DD until
there is no more energy in the DC link capacitor. The
protection is reset when V_DD drops to 2.5V, which is
allowed only after power supply is unplugged from the
AC line, as shown in Figure 38.
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