TOP412/414
TOPSwitch Family Functional Description
TOPSwitchis a self biased and protected linear control current-
to-duty cycle converter with an open drain output. High
efficiencyisachievedthrough theuseofCMOSandintegration
of the maximum number of functions possible. CMOS
significantly reduces bias currents as compared to bipolar or
discretesolutions. Integrationeliminatesexternalpowerresistors
used for current sensing and/or supplying initial start-up bias
current.
Auto-restart
I
B
D
MAX
Slope = PWM Gain
-16%/mA
During normal operation, the internal output MOSFET duty
cyclelinearlydecreaseswithincreasingCONTROLpincurrent
as shown in Figure 4. To implement all the required control,
bias,andprotectionfunctions,theDRAINandCONTROLpins
D
MIN
I
2.5
6.5
45
CD1
I
(mA)
C
PI-1691-112895
each perform several functions as described below. Refer to
Figure2forablockdiagramandFigure6fortimingandvoltage
waveforms of the TOPSwitch integrated circuit.
Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.
I
C
Charging C
T
5.7 V
4.7 V
Control Voltage Supply
V
CONTROL pin voltage VC is the supply or bias voltage for the
controller and driver circuitry. An external bypass capacitor
closely connected between the CONTROL and SOURCE pins
is required to supply the gate drive current. The total amount of
capacitance connected to this pin (CT) also sets the auto-restart
timing as well as control loop compensation. VC is regulated in
either of two modes of operation. Hysteretic regulation is used
for initial start-up and overload operation. Shunt regulation is
used to separate the duty cycle error signal from the control
circuit supply current. During start-up, VC current is supplied
fromahigh-voltageswitchedcurrentsourceconnectedinternally
between the DRAIN and CONTROL pins. The current source
provides sufficient current to supply the control circuitry as
well as charge the total external capacitance (CT).
C
0
Off
V
IN
0
DRAIN
Switching
(a)
I
CD2
Discharging C
I
I
T
C
CD1
Discharging C
Charging C
T
T
5.7 V
4.7 V
V
C
8 Cycles
95%
0
5%
Off
Off
Off
V
IN
0
DRAIN
Switching
Switching
(b)
C
is the total external capacitance
connected to the CONTROL pin
T
PI-1124A-060694
Figure 5. Start-up Waveforms for (a) Normal Operation and (b)
Auto-restart.
A
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POWERINT [ Power Integrations ]
