AAT2400 Datasheet, PDF (22/32 Pages) Skyworks Solutions Inc. – Sixteen-Channel White LED Driver Solution

DATA SHEET
AAT2400/2401
Sixteen-Channel White LED Driver Solution
with Full LED Current and Timing Control
Minimum and Maximum Practical Settings
for Gray Scale and Delay Registers
When using the internally generated PLL derived PWM
timebase, the stability of the frequency on V_SYNC defines
the practical minimum and maximum settings for the
gray scale and delay registers. As a rule, the minimum
and maximum settings should be 4 counts away from full
scale. In other words, the register settings should lie
between 04h and 0FFBh. Otherwise, the natural phase
delay between jitter on V_SYNC and the internal timebase
may result in unexpected PWM timing. Gray scale set to
zero, however, always results in zero on time.
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no recov-
ery, high voltage Schottky diodes are recommended for
the AAT2400 boost converter. The output diode is select-
ed to maintain acceptable efficiency and reasonable
operating junction temperature under full load operating
conditions. Forward voltage (V_F) and package thermal
resistance (θ_JA) are the dominant factors in selecting a
diode. The diode non-repetitive peak forward surge cur-
rent rating (I_FSM) should be considered for high pulsed
load applications. I_FSM rating drops with increasing con-
duction period.
Manufacturers’ datasheets should be reviewed carefully
to verify reliability under peak loading conditions. The
diode’s published current rating may not reflect actual
operating conditions and should be used only as a com-
parative measure between similarly rated devices.
60V/3A rated Schottky diodes are recommended for out-
put voltages less than 45V (total V_F and 100mA/sink
application).
Estimating Schottky
Diode Power Dissipation
The switching period is divided between ON and OFF
time intervals:
1
F_S ^{= tON + tOFF = D + D′}
During the ON time, the N-channel power MOSFET is
conducting and storing energy in the boost inductor.
During the OFF time, the N-channel power MOSFET is
not conducting. Stored energy is transferred 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
=
t_ON
t_ON + t_OFF
=
t_ON
·
F_S
The maximum duty cycle can be estimated from the
relationship for a continuous mode boost converter.
Maximum duty cycle (D_MAX) is the duty cycle at minimum
input voltage (V_IN(MIN)):
D_MAX =
V_OUT - V_IN(MIN)
V_OUT
The average diode current during the OFF time can be
estimated:
I_AVG(OFF)
=
1
I_OUT
- D_MAX
The V_F of the Schottky diode can be estimated from the
average current during the off time. The average diode
current is equal to the output current:
I_AVG(TOT) = I_OUT
The average output current multiplied by the forward
diode voltage determines the loss of the output diode:
P_LOSS(DIODE) = I_AVG(TOT) · V_F = I_OUT · V_F
For continuous LED currents, the diode junction tem-
perature can then be estimated:
T_J(DIODE) = T_AMB + θ_JA · P_LOSS(DIODE)
External Schottky diode junction temperature should be
below 110°C, and may vary depending on application
and/or system guidelines. The diode θ_JA can be mini-
mized with additional metal PCB area on the cathode.
However, adding additional heat-sinking metal around
the anode may degrade EMI performance. The reverse
leakage current of the rectifier must be considered to
maintain low quiescent (input) current and high effi-
ciency under light load. The rectifier reverse current
increases dramatically at elevated temperatures.
22
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