Datasheet

5
TNY263-268
G
4/05
V
DRAIN
V
EN
CLOCK
D
DRAIN
I
MAX
PI-2749-050301
Figure 6. TinySwitch-II Operation at Near Maximum Loading.
V
DRAIN
V
EN
CLOCK
D
DRAIN
I
MAX
PI-2667-090700
Figure 8. TinySwitch-II Operation at Medium Loading.
PI-2377-091100
V
DRAIN
V
EN
CLOCK
D
DRAIN
I
MAX
Figure 7. TinySwitch-II Operation at Moderately Heavy Loading.
the 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 applications with and without an external resistor (2 MΩ)
connected to the EN/UV pin.