TNY284-290
a Zener diode across the DC output voltage to be regulated.
When the output voltage exceeds the target regulation voltage
level (optocoupler LED voltage drop plus Zener voltage), the
optocoupler LED will start to conduct, pulling the ENABLE/
UNDERVOLTAGE pin low. The Zener diode can be replaced by
a TL431 reference circuit for improved accuracy.
At near maximum load, TinySwitch-4 will conduct during nearly
all of its clock cycles (Figure 7). At slightly lower load, it will
“skip” additional cycles in order to maintain voltage regulation at
the power supply output (Figure 8). At medium loads, cycles
will be skipped and the current limit will be reduced (Figure 9).
At very light loads, the current limit will be reduced even further
(Figure 10). Only a small percentage of cycles will occur to
satisfy the power consumption of the power supply.
ON/OFF Operation with Current Limit State Machine
The internal clock of the TinySwitch-4 runs all the time. At the
beginning of each clock cycle, it samples the ENABLE/
UNDERVOLTAGE pin to decide whether or not to implement a
switch cycle, and based on the sequence of samples over
multiple cycles, it determines the appropriate current limit. At
high loads, the state machine sets the current limit to its highest
value. At lighter loads, the state machine sets the current limit
to reduced values.
The response time of the ON/OFF control scheme is very fast
compared to PWM control. This provides tight regulation and
excellent transient response.
Power-Up/Down
The TinySwitch-4 requires only a 0.1 μF capacitor on the
BYPASS/MULTI-FUNCTION pin to operate with standard
200
200
V
V
100
100
DC-INPUT
DC-INPUT
0
0
10
10
V
V
5
5
BYPASS
BYPASS
0
0
400
400
200
200
V
V
DRAIN
DRAIN
0
0
1
2
0
1
2
0
Time (ms)
Time (ms)
Figure 12. Power-Up without Optional External UV Resistor
Connected to EN/UV Pin.
Figure 11. Power-Up with Optional External UV Resistor (4 MW)
Connected to EN/UV Pin.
200
200
V
V
100
0
DC-INPUT
100
0
DC-INPUT
400
300
400
300
V
200
100
0
V
200
100
0
DRAIN
DRAIN
2.5
5
0
.5
1
0
Time (s)
Time (s)
Figure 14. Slow Power-Down Timing with Optional External (4 MW)
Figure 13. Normal Power-Down Timing (without UV).
UV Resistor Connected to EN/UV Pin.
6
Rev. A 09/12
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