Specifications

Rev. 1.0 January 2012
www.aosmd.com
Page 12 of 12
AOZ197
8
Application Information
Inductor Selection
Inductor choice will be affected by many parameters,
like duty cycle based on input/output setting, switching
frequency, full scale LED current level, and mode of
operations. 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 best stability and good efficiency.
Inductor Current in Critical Conduction Mode
The inductor value is determined by:
For the application with VIN=100V, VOUT=200V, LED
current=125mA.
A
V
AV
V
IV
ICurrentInput
MININ
OUTOUT
IN
29.0
85
125.0200
_
)(





In critical Conduction mode inductor Peak current:
AIdiIL
INPEAK
58.02 
5.0
200
100200
_ 




V
VV
V
VV
DCycleDuty
OUT
INOUT
s
kHzF
D
dttimeON
SW
 5.2
200
5.0
_ 
H
Vs
di
Vdt
L
MININ


370
58.0
855.2
)(





After the inductor value is calculated, we need to
consider the DCR resistance and the saturation
current I
SAT
of the inductor. Inductor DCR is inversely
proportional to the I
SAT
. It is recommended to select an
inductor for which the I
SAT
value should be at least
50% higher than the I
LPEAK
value. To minimize 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 rating 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 (V
RIPPLE
)
requirement:
SWRIPPLE
OUT
OUT
F
D
V
I
C 
When selecting output capacitors, it is more important
to check the effective ESR of the capacitor than the
actual capacitance value. For example, a 10μF
capacitor with 0.02Ω ESR will handle higher ripple
current and produce less output ripple than a 33μF
capacitor with 0.04Ω 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 and does not contain large triangular
peak current as compared to the output capacitors.
AOZ1978 utilizes 180⁰ out of phase switching which
provides input ripple current cancellation and
significantly reduces input capacitance requirements
compared to in-phase switching.
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.
Current Sense Resistors
There are three current sense resistors in this
application, an LED Feedback current sense resistor
RFB, a Boost switch Current Sense resistor RCS and
an optional system Ground return over current
protection R
RTN
.
RFB LED current sense resistor is set by:
 4
125.0
5.0
_ A
V
CURRENTLED
V
RFB
ISET
RCS boost switch Current Sense resistor is set by:




 4.0
3.158.0
3.0
3.1
3.0
A
V
IL
V
RCS
PEAK
IL
PEAK