LT3498
APPLICATIONS INFORMATION—LED DRIVER
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 6 is approximately 40µs for a
3V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
The dimming range can be further extended by changing
the amplitude of the PWM signal. The height of the PWM
signalsetsthecommandedsensevoltageacrossthesense
resistorthroughtheCTRL1pin.Inthismannerbothanalog
dimming and direct PWM dimming extend the dimming
range for a given application. The color of the LEDs no
longer remains constant because the forward current of
the LED changes with the height of the CTRL1 signal. For
the four LED application described above, the LEDs can
be dimmed first, modulating the duty cycle of the PWM
signal.Oncetheminimumdutycycleisreached,theheight
of the PWM signal can be decreased below 1.5V down to
125mV. The use of both techniques together allows the
average LED current for the four LED application to be
variedfrom20mAdowntolessthan20µA. Figure9shows
the application for dimming using both analog dimming
and PWM dimming. A potentiometer must be added to
ensure that the gate of the NMOS receives a logic-level
signal, while the CTRL1 signal can be adjusted to lower
amplitudes.
Example:
f = 100Hz, tSETTLE = 40µs
1
f
1
100
tPERIOD = =
= 0.01s
0.01s
tSETTLE 40µs
tPERIOD
DimRange =
=
= 250 :1
MinDuty Cycle =
40µs
0.01s
tSETTLE
tPERIOD
• 100 =
• 100 = 0.4%
Duty Cycle Range = 100% → 0.4% at 100Hz
V
IN
3V TO 5V
C
IN
1µH
R
Thecalculationsshowthatfora100Hzsignalthedimming
range is 250:1. In addition, the minimum PWM duty cycle
of 0.4% ensures that the LED current has enough time to
settle to its final value. Figure 8 shows the dimming range
achievable for three different frequencies with a settling
time of 40µs.
SENSE1
10Ω
L1
15µH
CAP1 SW1
V
SW2 CAP2 V
IN OUT2
LT3498
C
OUT1
1µF
LED1
CTRL1
CTRL2
FB2
GND1
GND2
5V
0V
10000
PWM
FREQ
3498 F09
PULSING MAY BE VISIBLE
1000
100
10
Q1
Si2304BDS
100k
Figure 9. Li-Ion to Four White LEDs with
Both PWM Dimming and Analog Dimming
1
10
100
1000
10000
PWM FREQUENCY (Hz)
3498 F08
Figure 8. Dimming Ratio vs Freqeuncy
3498f
13
Linear [ Linear ]
