TNY263-268
SOURCE pin. The optocoupler LED is connected in series with
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 EN/UV pin
low. The Zener diode can be replaced by a TL431 reference
circuit for improved accuracy.
ON/OFF Operation with Current Limit State Machine
The internal clock of the
TinySwitch-II
runs all the time. At the
beginning of each clock cycle, it samples the EN/UV 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, when the EN/UV
pin is high (less than 240
µA
out of the pin), a switching cycle
with the full current limit occurs. At lighter loads, when EN/UV
is high, a switching cycle with a reduced current limit occurs.
At near maximum load,
TinySwitch-II
will conduct during
nearly all of its clock cycles (Figure 6). At slightly lower load,
it will “skip” additional cycles in order to maintain voltage
regulation at the power supply output (Figure 7). At medium
loads, cycles will be skipped and the current limit will be
reduced (Figure 8). At very light loads, the current limit will be
reduced even further (Figure 9). Only a small percentage of
cycles will occur to satisfy the power consumption of the power
supply.
The response time of the
TinySwitch-II
ON/OFF control scheme
is very fast compared to normal PWM control. This provides
tight regulation and excellent transient response.
Power Up/Down
The
TinySwitch-II
requires only a 0.1
µF
capacitor on the
BYPASS pin. Because of its small size, the time to charge this
capacitor is kept to an absolute minimum, typically 0.6 ms. Due
to the fast nature of the ON/OFF feedback, there is no overshoot
at the power supply output. When an external resistor (2 MΩ) is
connected from the positive DC input to the EN/UV pin, the power
MOSFET switching will be delayed during power-up until the DC
line voltage exceeds the threshold (100 V). Figures 10 and 11
show the power-up timing waveform of
TinySwitch-II
in
V
EN
CLOCK
D
MAX
I DRAIN
V DRAIN
PI-2749-050301
Figure 6. TinySwitch-II Operation at Near Maximum Loading.
V
EN
CLOCK
V
EN
CLOCK
D
MAX
D
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2667-090700
PI-2377-091100
Figure 7. TinySwitch-II Operation at Moderately Heavy Loading.
Figure 8. TinySwitch-II Operation at Medium Loading.
E
4/04
5
POWERINT [ POWER INTEGRATIONS, INC. ]
