English
Language : 

AAT1235 Datasheet, PDF (15/21 Pages) Advanced Analogic Technologies – High Efficiency White LED Drivers for Backlight and Keypad
◁
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
LED Brightness Control
The AAT1235 uses AS2Cwire programming to con-
trol LED brightness. The output current of the
AAT1235 can be changed successively to brighten
or dim the LEDs in smooth transitions (i.e., to fade
in or fade out) or in discrete steps, giving the user
complete programmability and real-time control of
LED brightness.
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no
recovery, high voltage Schottky diodes are recom-
mended for the AAT1235 boost converter. The out-
put diode is selected to maintain acceptable effi-
ciency and reasonable operating junction tempera-
ture under full load operating conditions. Forward
voltage (VF) and package thermal resistance (θJA)
are the dominant factors in selecting a diode. The
diode’s non-repetitive peak forward surge current
rating (IFSM) should be considered for high pulsed
load applications, such as camera flash. IFSM rating
drops with increasing conduction period.
Manufacturers’ datasheets should be reviewed
carefully to verify reliability under peak loading con-
ditions. The diode's published current rating may
not reflect actual operating conditions and should
be used only as a comparative measure between
similarly rated devices.
20V rated Schottky diodes are recommended for
output voltages less than 15V, while 30V rated
Schottky diodes are recommended for output volt-
ages higher than 15V.
Estimating Schottky Diode Power
Dissipation
The switching period is divided between ON and
OFF time intervals:
1
FS
=
TON
+
TOFF
During the ON time, the N-channel power MOSFET
is conducting and storing energy in the boost induc-
tor. During the OFF time, the N-channel power
MOSFET is not conducting. Stored energy is trans-
ferred from the input battery and boost inductor to
the output load through the output diode.
Duty cycle is defined as the ON time divided by the
total switching interval:
D=
TON
TON + TOFF
= TON ⋅ FS
The maximum duty cycle can be estimated from
the relationship for a continuous mode boost con-
verter. Maximum duty cycle (DMAX) is the duty cycle
at minimum input voltage (VIN(MIN)):
DMAX =
VOUT - VIN(MIN)
VOUT
The average diode current during the OFF time can
be estimated:
IAVG(OFF)
=
1
IOUT
- DMAX
The VF of the Schottky diode can be estimated from
the average current during the off time. The aver-
age diode current is equal to the output current:
IAVG(TOT) = IOUT
The average output current multiplied by the for-
ward diode voltage determines the loss of the out-
put diode:
PLOSS(DIODE) = IAVG(TOT) · VF
= IOUT · VF
1235.2007.02.1.1
15
 ▷