AAT1235 Analogic Corporation, AAT1235 Datasheet - Page 15

• Manufacturer Part Number: AAT1235
• Manufacturer: Analogic Corporation

LED Brightness Control
The AAT1235 uses AS
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 (V
are the dominant factors in selecting a diode. The
diode’s non-repetitive peak forward surge current
rating (I
load applications, such as camera flash. I
drops
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:
During the ON time, the N-channel power MOSFET
is conducting and storing energy in the boost induc-
1235.2007.01.1.0
FSM
with
F
) should be considered for high pulsed
) and package thermal resistance (θ
increasing
F
1
S
= T
2
Cwire programming to con-
ON
+ T
conduction
OFF
FSM
period.
rating
JA
)
High Efficiency White LED Drivers
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:
The maximum duty cycle can be estimated from
the relationship for a continuous mode boost con-
verter. Maximum duty cycle (D
at minimum input voltage (V
The average diode current during the OFF time can
be estimated:
The V
the average current during the off time. The aver-
age diode current is equal to the output current:
The average output current multiplied by the for-
ward diode voltage determines the loss of the out-
put diode:
F
of the Schottky diode can be estimated from
for Backlight and Keypad
P
LOSS(DIODE)
D
MAX
I
AVG(OFF)
D =
I
AVG(TOT)
=
= T
T
V
ON
ON
= I
= I
=
OUT
T
⋅ F
1 - D
= I
OUT
+ T
AVG(TOT)
V
ON
- V
IN(MIN)
I
OUT
S
OUT
OUT
OFF
· V
MAX
IN(MIN)
MAX
F
) is the duty cycle
· V
):
F
15