FA5546 Datasheet, PDF (17/27 Pages) Fuji Electric – Switching Power Supply Control IC

FA5546/47
(5) Overload protection circuit (auto restart type)
Detecting overload, the overload protection (OLP) circuit
stops switching in case overload is continued for a specified
period of time (200ms typ) to protect the circuit.
Overload detection is performed based on the FB terminal
voltage.
The overload detection level has been set to lower than the
maximum input threshold of the IS terminal (2-stage OLP).
Consequently, overcurrent can be output while the output
voltage is maintained within specified time (200ms typ), on
condition that the overload current does not exceed the
maximum power threshold determined by the IS terminal. It
is therefore ideal for applications in which overcurrent is fed
instantaneously such as motors.
Fig.10 is a conceptual diagram of the operation.
Fig.11 shows a circuit block, and Fig.12 shows the
protection operation timing chart, of the auto reset type.
The overload protection is actuated when FB terminal
voltage processed into a signal is detected with the OLP
comparator, based on the overload delay time TdOLP
(200ms typ), which is the period from the overload
detection with the built-in OLP timer to switching pulse stop,
and the auto reset wait time TdOLP2 (1400ms typ), which
is the period from switching stop to switching restart. These
periods are specified within the IC.
When the output current increases and the FB terminal
voltage increases up to VthOLP (2.8V typ), overload is
detected. In this case, since the IS terminal voltage is
equivalent to VthIS2 (-0.63V typ), which is smaller than the
maximum input threshold voltage, VthIS1 (-0.95V typ), the
MOSFET current is not limited and the output voltage is
maintained. If the output current is further increased and
the IS terminal voltage increases to reach the maximum
threshold voltage, VthIS1 (-0.95V typ), the MOSFET
current is limited and the output voltage cannot be
maintained any longer.
As soon as the overload is detected, the OLP timer starts
counting the delay time up to the stop of the switching
pulse.
When specified overload protection delay time TdOLP
(200ms typ) has elapsed, the switching pulse output is
stopped. If overload is reset and the FB voltage decreases
down to VthOLP (2.8V typ) before the overload protection
delay time expires, switching continues.
switching is suspended, and the VCC voltage is kept within
the 15.5V to 18V (typ) range.
When auto reset wait time TdOLP2 has elapsed, the
switching pulse is output for rebooting.
Output Voltage
Output Current
MOSFET
Gate
Vo
0
Current limit
Over load detection
0
VCC
0
OLP period Stop
over 200 ms ^switching
Fig. 10 Conceptual diagram of overload protection
operation
Start up
Start up VCC
Management
8
Current
VH
Logic
Monitor Ctrl
ON
ON
DBL
5
OUT
5V
Rfb
FB ^20kΩ
2
INV.
AMP
A
Over
OLP
Load
T1
Ctrl
T2
OLP Timer
IS comp.
VthIS1
4
3
GND
IS
Fig.11 Overload protection circuit (auto recovery)
When switching is stopped, the OLP timer counts the auto
reset wait time TdOLP2 (1400ms typ).
The startup circuit is kept under the ON/OFF control while
Fuji Electric Co., Ltd.
AN-027E Rev.0.4
17
April -2011
http://www.fujielectric.co.jp/products/semiconductor/