AN-6073 查看數據表（PDF） - Fairchild Semiconductor
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AN-6073 Datasheet PDF : 13 Pages
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AN-6073
If the conversion efficiency remains unchanged for a wide
input voltage range, the maximum output power would be
the same for a fixed IPK, which is limited by the internal
current limiting threshold voltage VTH and RS. However, due
to the time delay from the comparator to output stage inside
the FAN6751, the maximum output power with high-line
input is always higher than with low line. A 30% error is
common for the universal input voltage range if the
converter is operated in DCM. In CCM operation, the
deviation becomes even larger. For the purpose of constant
output power limit, the peak current limit VTH must be
adjustable according to the VIN pin, which is proportional
to input voltage. VIN=1V and VSENSE=0.83V at low line;
VIN=3V and VSENSE=0.7 at high line.
APPLICATION NOTE
or Latched (for HL). OVP condition is usually caused by
feedback open loops.
Overload Protection (OLP)
If the secondary output short circuits or the feedback loop is
open, the FB pin voltage rises rapidly toward the open-loop
voltage, VFB-OPEN. If the FB voltage remains above VFB-OLP
and lasts for tD-OLP, the FAN6751 stops emitting output
pulses and enters Recovery Mode (for FAN6751MR) or
latched-up mode (for FAN6751HL), as shown in Figure 17.
Vds
Power
On
OVP
occurs
Removed
AC line
Power
On
Figure 14. Universal Line Voltage Compensation for
Constant Output Power Limit
Protection Functions
FAN6751 has protection functions in two categories: some
enter Latch Mode and the others enter Recovery Mode. The
Latch Mode can only be restart if VDD falls below 5V, as
shown Figure 15. The Recovery Mode lets VDD decrease to
UVLO mechanism until the fault condition removed, as
shown Figure 16. Both modes prevent the SMPS from
destructive states. Table 2 shows the relationship between
protection functions and part numbers.
VDD
26V
16.5V
10.5V
7.5V
5V
Normal Latch mode
operation Without any
switching
Remove
Latch
mode
Normal
operation
Figure 15.VDD OVP Protection Waveforms
for FAN6751HL, Latch Off
Table 2. Protection Functions
Protection Functions FAN6751MR FAN6751HL
VDD Over-Voltage
Protection (OVP)
Overload Protection (OLP)
Pull-High Protection
Function by VIN > 4.7V
Internal Over Temperature
Protection(OTP)
SENSE Pin Short-Circuit
Protection
Brownout Protection
Recovery
Recovery
Recovery
Recovery
Recovery
Recovery
Latch
Latch
Latch
Recovery
Recovery
Recovery
VDD Over-Voltage Protection (OVP)
VDD OVP has protection prevents damage due to over-
voltage conditions. When the VDD voltage exceed 26V due
to abnormal conditions, PWM output is turned off until the
VDD voltage drops below UVLO then starts again (for MR)
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/26/08
6
Figure 16. VDD OVP Protection Waveforms for
FAN6751MR, Recovery
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