Datasheet
IN EST
IN
OUT LIM
PRI SW OUT
V min Dmax
V min Dmax
I max I
2 L f V
§ ·
u u K
u
u
¨ ¸
u u
© ¹
IN OUT D
FET
V max V V N
V
0.8
u
OUT D
CCM
IN OUT D
V V N
D
V V V N
u
u
OUT
REF
V
R8 R14 1
V
§ ·
u
¨ ¸
© ¹
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Test Setup and Results
(1)
Note that V
IN
must be in a range so that the on-time is greater than the minimum controllable on-time
(77ns typical), and the maximum duty cycle is less than 89% minimum and 93% typical. Additionally, for a
flyback application it is recommended to limit the duty cycle to 50% max to reduce stress on external
components and avoid sub-harmonic operation inherent to current mode control. The duty cycle in
continuous-conduction mode (CCM) can be calculated with Equation 2, where V
D
is the forward voltage
drop of the secondary Schottky diode (D1) and N is the primary to secondary turns ratio of the
transformer.
(2)
After adjusting the output voltage, the maximum input voltage must be taken into consideration. The
voltage on the internal FET, the SW pin, is equal to the sum of the input voltage and the output voltage
reflected through the transformer. Additional tolerance is recommended to accommodate ringing typical of
a flyback supply. By using Equation 3, you can estimate the voltage across the internal FET with a 20%
tolerance. This must be less than the 40V absolute maximum rating.
(3)
3.2 Maximum Output Current
After adjusting input or output voltage settings, verify the maximum output current per Equation 4 below,
where I
LIM
is the peak current limit of the TPS55340, Dmax is the CCM duty cycle with the minimum input
voltage and η
EST
is the estimated maximum load current efficiency.
(4)
3.3 Soft-start Time
The primary soft-start time can be adjusted by changing the value of C12. The EVM uses C12 = 0.1 µF to
control the ramp of the COMP pin voltage. A larger capacitance increases the soft-start time while a
smaller capacitance decreases it. Additionally C18 provides a soft-start for the secondary side reference
based on the RC time constant of C18 and R6. The EVM uses C18 = 1.0 µF to avoid any overshoot
during start up. It is recommended to have the soft-start for the secondary side reference be longer than
the primary side soft-start to avoid overshoot. A higher RC time constant increases the soft-start time
while a lower RC time constant decreases it.
3.4 Other Modifications
When changing the switching frequency, input/output voltage range, transformer, output capacitors or
compensation additional design changes may be needed. When making design changes it is
recommended to replace the transformer with one optimized for the application's requirements for best
performance.
4 Test Setup and Results
This section describes how to properly connect, set up, and use the TPS55340EVM-148 evaluation
module. Included are test results typical for the evaluation module covering efficiency, output voltage
regulation, load transients, loop response, output voltage ripple, input voltage ripple, start-up and shut-
down. Measurements are for an ambient temperature of 25°C.
3
SLVU751A–July 2012–Revised June 2013 TPS55340EVM-148, 8V to 24V Input, 5V Output Flyback Evaluation Module
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