TOP221-227
V
IN
V
IN
0
DRAIN
V
OUT
0
0
I
OUT
1
1
2
2
8
8
1
1
2
2
8
8
1
1
• • •
• • •
• • •
• • •
V
V
C
C(reset)
0
0
I
C
1
3
1
1
2
PI-2030-042397
Figure 6. Typical Waveforms for (1) Normal Operation, (2) Auto-restart, and (3) Power Down Reset.
the output MOSFET off until the start of the next clock cycle.
The current limit comparator threshold voltage is temperature
compensatedtominimizevariationoftheeffectivepeakcurrent
limit due to temperature related changes in output MOSFET
When the fault condition is removed, the power supply output
becomes regulated, VC regulation returns to shunt mode, and
normal operation of the power supply resumes.
RDS(ON)
.
Overtemperature Protection
Temperature protection is provided by a precision analog
circuit that turns the output MOSFET off when the junction
temperature exceeds the thermal shutdown temperature
(typically 135 °C). Activating the power-up reset circuit by
removingandrestoringinputpowerormomentarilypullingthe
CONTROL pin below the power-up reset threshold resets the
latch and allows TOPSwitch to resume normal power supply
operation. VC is regulated in hysteretic mode and a 4.7 V to
5.7V(typical)sawtoothwaveformispresentontheCONTROL
pin when the power supply is latched off.
The leading edge blanking circuit inhibits the current limit
comparator for a short time after the output MOSFET is turned
on. The leading edge blanking time has been set so that current
spikescausedbyprimary-sidecapacitancesandsecondary-side
rectifier reverse recovery time will not cause premature
termination of the switching pulse.
Thecurrentlimitcanbelowerforashortperiodaftertheleading
edge blanking time as shown in Figure 12. This is due to
dynamic characteristics of the MOSFET. To avoid triggering
thecurrentlimitinnormaloperation,thedraincurrentwaveform
should stay within the envelope shown.
High-voltage Bias Current Source
ThiscurrentsourcebiasesTOPSwitchfromtheDRAINpinand
charges the CONTROL pin external capacitance (CT) during
start-up or hysteretic operation. Hysteretic operation occurs
during auto-restart and overtemperature latched shutdown.
The current source is switched on and off with an effective duty
cycle of approximately 35%. This duty cycle is determined by
the ratio of CONTROL pin charge (IC) and discharge currents
(ICD1 and ICD2). This current source is turned off during normal
operation when the output MOSFET is switching.
Shutdown/Auto-restart
To minimize TOPSwitch power dissipation, the shutdown/
auto-restart circuit turns the power supply on and off at an auto-
restart duty cycle of typically 5% if an out of regulation
condition persists. Loss of regulation interrupts the external
current into the CONTROL pin. VC regulation changes from
shuntmodetothehystereticauto-restartmodedescribedabove.
C
5
12/97
POWERINT [ Power Integrations ]
