AOZ1977
Application Information
DutyCycle = D = ----------------------------
Iripple = ---------------------------------
ONtime = dt = -----------
D = ---------------------------------- = 0.28
dt = --------------------- = 2.8 us
0.3 ? Vin ? ? Vout – Vin ?
Fsw ? L ? Vout
dt ? Vin 2.8 us ? 130 V
L = --------------------- = ------------------------------------ = 656 uH
Inductor Selection
Inductor choice will be affected by many parameters,
including duty cycle based on input/output setting,
switching frequency, full scale LED current level, and
mode of operations. The boost controller can operate
under discontinuous mode, continuous mode, or critical
conduction mode. For high voltage boost LED driver
applications, it is recommended to use critical conduction
mode for good stability and best efficiency.
ILpeak
Inductor Current in Critical Conduction Mode
In critical conduction mode:
ILpeak = di = 2 ? Iin
The duty cycle for the boost DC/DC system is defined as:
Vout – Vin
V OUT
To determine the ON time for the boost switch:
D
Fsw
For the application with VIN = 130 V, VOUT = 180 V,
LED current = 200mA:
- 180 V ? 0.2 A
lin = -------------------------------- = 0.277 A
130 V
di = 2 ? 0.277 A = 0.555 A
180 V – 130 V
180 V
0.28
100 kHz
The inductor value is determined by:
di 0.555 A
After the inductor value is calculated, we need to
consider the DCR resistance and the Isat saturation
current of the inductor. Inductor DCR is inversely
proportional to the Isat. It is recommended to select an
inductor for which the Isat value should be at least 50 %
higher than the ILpeak value. To minimize the EMI effect,
it is always preferable to use shielded type inductors.
Diode Selection
It is recommended to use fast recovery diode for D1.
For most applications, Schottky diodes with correct
current and voltage ratings are suitable. The diode
current rating should be at least higher than the full scale
LED current. The diode voltage rating should be higher
than the OVP level of VOUT voltage.
Output Capacitors
The amount and type of capacitor used is mainly
determined by the design output ripple requirement, and
mainly by the output ripple current which is usually higher
for boost converters and equals:
Vout – Vin
Vin
When selecting output capacitors, it is more important to
check the effective ESR of the capacitor than the actual
capacitance value. For example, a 10 ? F capacitor with
0.02 ? ESR will handle higher ripple current but produce
less output ripple than a 33 ? F capacitor with 0.04 ?
ESR. It is recommended to use low ESR MLCC ceramic
capacitors. For high voltage cost effective application,
multiple Electrolytic capacitors in parallel will reduce the
total effective ESR.
Input Capacitors
The input capacitors for boost converters do not require
low ESR due to the fact that the input current is
continuous. Also, they do not contain large peak current
as compared to the output capacitors.
The ripple current at the input capacitor is:
Iin_ripple = ---------------------------------------------------------------
where Fsw is the switching frequency, 100 kHz in this
example.
Electrolytic capacitors should work well with the
appropriate voltage and ripple current rating, it is not
recommended to use Tantalum capacitors because
Boost converters do exhibit high surge currents during
startup which can cause tantalum capacitors to fail.
Rev. 2.1 May 2012
www.aosmd.com
Page 11 of 16
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