LT3598
12
3598fb
APPLICATIONS INFORMATION
For True Color PWM
TM
dimming, the LT3598 provides up
to a 3000:1 PWM dimming range. This is achieved by
allowing the duty cycle of the PWM pin (connected to
the IC and an internal switch in series with the LED(s)),
to be reduced from 100% to as low as 0.1% for a PWM
frequency of 100Hz (Figure 6). PWM duty cycle dimming
allows for constant LED color to be maintained over the
entire dimming range.
For wide PWM dimming range, higher switching freq-
uency and lower PWM frequency confi guration are
needed. Special considerations are required for component
selection and compensation network. Please contact
factory for optimized components selection if very high
dimming ratio is desired.
LED Current Derating Using the CTRL Pin
A useful feature of the LT3598 is its ability to program
a derating curve for maximum LED current versus
temperature. LED data sheets provide curves of maximum-
allowable LED current versus temperature to warn against
exceeding this current limit and damaging the LED. The
LT3598 allows the output LEDs to be programmed for
maximum allowable current while still protecting the
LEDs from excessive currents at high temperature. This
is achieved by programming a voltage at the CTRL pin
with a negative temperature coeffi cient using a resistor
divider with temperature dependent resistance (Figure 7).
As ambient temperature increases, the CTRL voltage will
fall below the internal 1V voltage reference, causing LED
currents to be controlled by the CTRL pin voltage. The LED
current curve breakpoint and slope versus temperature
is defi ned by the choice of resistor ratios and use of
temperature-dependent resistance in the divider for the
CTRL pin.
Table 5 shows a list of manufacturers/distributors of
NTC resistors. There are several other manufacturers
available and the chosen supplier should be contacted
for more detailed information. If an NTC resistor is used
to indicate LED temperature, it is effective only if the
resistor is connected as closely as possible to the LED
strings. LED derating curves shown by manufacturers are
listed for ambient temperature. The NTC resistor should
have the same ambient temperature as the LEDs. Since
the temperature dependency of an NTC resistor can be
nonlinear over a wide range of temperatures, it is important
to obtain a resistor’s exact value over temperature from
the manufacturer. Hand calculations of CTRL voltage can
then be performed at each given temperature, resulting
in the CTRL versus temperature plotted curve. Several
iterations of resistor value calculations may be required
to achieve the desired breakpoint and slope of the LED
current derating curve.
Figure 4. LED Current vs CTRL Voltage
Figure 5. LED Current vs CTRL
Figure 6. LED Current Using PWM Dimming
3598 F05
LT3598
V
REF
CTRL
R2
R1
PWM
INDUCTOR
CURRENT
3598 F06
LED
CURRENT
MAX I
LED
T
PWM
TON
PWM
(= 1/f
PWM
)
CTRL (V)
0
LED CURRENT (mA)
20
25
30
1.4
15
10
0.4 0.8
0.2
0.6 1 1.2 1.6
5
0
35
3598 F04
R
ISET
= 9.76k
