TNY263-268
the SOURCE pin. The optocoupler LED is connected in series
withaZenerdiodeacrosstheDCoutputvoltagetoberegulated.
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.
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 lighterloads, when EN/UV
is high, a switching cycle with a reduced current limit occurs.
ON/OFF Operation with Current Limit State Machine
The internal clock of the TinySwitch-II runs all the time. At
Atnearmaximumload,TinySwitch-IIwillconductduringnearly
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).
Atverylightloads,thecurrentlimitwillbereducedevenfurther
(Figure 9). Only a small percentage of cycles will occur to
satisfy the power consumption of the power supply.
V
EN
CLOCK
TheresponsetimeoftheTinySwitch-IION/OFFcontrolscheme
is very fast compared to normal PWM control. This provides
tight regulation and excellent transient response.
D
MAX
I
Power Up/Down
DRAIN
The TinySwitch-II requires only a 0.1 µF capacitor on the
BYPASS pin. Because of its small size, the time to charge this
capacitoriskepttoanabsoluteminimum, typically0.6ms. Due
tothefastnatureoftheON/OFFfeedback, thereisnoovershoot
at the power supply output. When an external resistor (2 MΩ)
is connected from the positive DC input to the EN/UV pin, the
powerMOSFETswitchingwillbedelayedduringpower-upuntil
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.
V
DRAIN
PI-2749-050301
Figure 6. TinySwitch-II Operation at Near Maximum Loading.
V
V
EN
EN
CLOCK
CLOCK
D
D
MAX
MAX
I
I
DRAIN
DRAIN
V
DRAIN
V
DRAIN
PI-2667-090700
PI-2377-091100
Figure 8. TinySwitch-II Operation at Medium Loading.
Figure 7. TinySwitch-II Operation at Moderately Heavy Loading.
G
4/05
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POWERINT [ Power Integrations ]
