Texas Instruments DRV2700EVM-HV500, User Manual, Page: 14 / 30

REF
EXT INT
LIM
V
R K R
I
æ ö
= -
ç ÷
è ø
Flyback Converter
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4.2 Programming the Flyback Current Limit
The peak inductor current is set with resistor R3 (R
EXT
). The current limit is not a safety mechanism, but
the highest value current the inductor will see each cycle. The inductor must be capable of handling this
programmed limit during normal operation. The relationship of R
EXT
to I
LIM
is approximated with Equation 3
where I
LIM
is the current limit set by R
EXT
, K = 10500, V
REF
= 1.35 V and R
INT
= 60 Ω .
(3)
4.3 Transformer Selection
Transformer selection plays a critical role in the performance of the DRV2700. The range of
recommended primary-side inductance values is 3.3 µH to 22 µH. When a larger inductance is chosen,
the DRV2700 flyback converter automatically runs at a lower switching frequency and incurs less
switching losses; however, the larger inductances may also have a higher equivalent series resistance
(ESR), which will increase the parasitic inductor losses.
Another factor to consider for transformers is the winding ratio. In general, if a 200-V output is desired
then, because the SW node can boost up to 100 V, a transformer of 1:2 (100 V:200 V) is the minimum
required winding. However, selecting a slightly higher winding ratio to ensure that the 100 V on the
primary side is not surpassed while trying to boost up to the desired voltage is good design practice.
The transformer used on this EVM is a 1:10 winding ratio with a primary side inductance of 7 µH.
4.4 HV Capacitor Selection
The HV output voltage may be programmed as high as 500 V on this EVM. A capacitor must have a
voltage rating equivalent to the boost output voltage or higher. Because the output can be unloaded, a 1-
nF output capacitor is added to ensure some amount of stability on the output.
Additionally, a non-populated landing pad (C6) is provided for additional capacitance, if desired.
14 DRV2700EVM-HV500 High Voltage Piezo Driver Evaluation Kit SLOU407A – April 2015 – Revised May 2015
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