Figure 14. 2.5 W Constant Voltage, Constant Current Battery Charger with Universal Input (85-265 VAC).
reducing the no-load power consumption and improving full-
load efﬁ ciency.
Current Limit Operation
Each switching cycle is terminated when the DRAIN current
reaches the current limit of the TinySwitch-II. Current limit
operation provides good line ripple rejection and relatively
constant power delivery independent of input voltage.
BYPASS Pin Capacitor
The BYPASS pin uses a small 0.1 μF ceramic capacitor for
decoupling the internal power supply of the TinySwitch-II.
The TinySwitch-II is ideal for low cost, high efﬁ ciency power
supplies in a wide range of applications such as cellular phone
chargers, PC standby, TV standby, AC adapters, motor
control, appliance control and ISDN or a DSL network
termination. The 132 kHz operation allows the use of a low
cost EE13 or EF12.6 core transformer while still providing good
efﬁ ciency. The frequency jitter in TinySwitch-II makes it
possible to use a single inductor (or two small resistors for
under 3 W applications if lower efﬁ ciency is acceptable) in
conjunction with two input capacitors for input EMI ﬁ ltering.
The auto-restart function removes the need to oversize the
output diode for short circuit conditions allowing the design to
be optimized for low cost and maximum efﬁ ciency. In charger
applications, it eliminates the need for a second optocoupler
and Zener diode for open loop fault protection. Auto-restart
C8 680 pF
also saves the cost of adding a fuse or increasing the power
rating of the current sense resistors to survive reverse battery
conditions. For applications requiring undervoltage lock out
(UVLO), such as PC standby, the TinySwitch-II eliminates
several components and saves cost. TinySwitch-II is well
suited for applications that require constant voltage and
constant current output. As TinySwitch-II is always powered
from the input high voltage, it therefore does not rely on bias
winding voltage. Consequently this greatly simpliﬁ es designing
chargers that must work down to zero volts on the output.
2.5 W CV/CC Cell-Phone Charger
As an example, Figure 14 shows a TNY264 based 5 V,
0.5 A, cellular phone charger operating over a universal input
range (85 VAC to 265 VAC). The inductor (L1) forms a π-ﬁ lter
in conjunction with C1 and C2. The resistor R1 damps
resonances in the inductor L1. Frequency jittering operation of
TinySwitch-II allows the use of a simple π-ﬁ lter described
above in combination with a single low value Y1-capacitor (C8)
to meet worldwide conducted EMI standards. The addition of
a shield winding in the transformer allows conducted EMI to be
met even with the output capacitively earthed (which is the
worst case condition for EMI). The diode D6, capacitor C3
and resistor R2 comprise the clamp circuit, limiting the leakage
inductance turn-off voltage spike on the TinySwitch-II DRAIN
pin to a safe value. The output voltage is determined by the
sum of the optocoupler U2 LED forward drop (~1 V), and Zener
diode VR1 voltage. Resistor R8 maintains a bias current
through the Zener diode to ensure it is operated close to the
Zener test current.
+ 5 V
Rev. H 02/09