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LTM8042/LTM8042-1
APPLICATIONS INFORMATION
Which mode to use depends upon the operating condi-
tions. Boost is generally selected when the voltage across
the LED string is always higher than the input voltage. Buck
mode is the dual of boost, used when the voltage across
the LED string is always lower than V IN . Finally, buck-boost
mode is used when the V IN can vary both above and below
the voltage across the LED string.
The land grid array of the LTM8042/LTM8042-1 is designed
to conveniently accommodate all three operating modes.
Please refer to the PCB Layout section for suggested
examples of how to lay out each operating mode.
The minimum PWM on or off-time will depend on the
choice of operating frequency through the RT input pin or
the SYNC pin. When using the SYNC function, the SYNC
and PWM signals must have the rising edges aligned to
achieve the optimized high PWM dimming ratio. For best
current accuracy, the minimum PWM low or high time
should be at least six switching cycles (3μs for ? SW =
2MHz). The maximum PWM period is determined by the
system. The maximum PWM dimming ratio (PWM RATIO )
can be calculated from the maximum PWM period (T MAX )
and the minimum PWM pulse width (T MIN ) as follows:
Dimming Control
PWM RATIO =
T MAX
T MIN
(1)
There are two methods to control the current source for
dimming using the LTM8042/LTM8042-1. One method
uses the PWM pin to modulate the current source between
zero and full current to achieve a precisely programmed
average current. To make this method of current control
more accurate, the switch demand current is internally
stored during the quiescent phase (PWM low). This feature
minimizes recovery time when the PWM signal returns
high. When using PWM dimming, use a P-channel MOSFET
disconnect switch in the LED current path (see Figure 1)
to prevent the output capacitor from discharging during
the PWM off-time. Enable this function by pulling TGEN
above 1.5V.
A set of values that give a 3000:1 dimming ratio, for ex-
ample, would be a switching frequency of ? SW = 2MHz,
T MAX = 9ms and T MIN = 3μs. Equation (1) becomes:
PWM RATIO = 9ms/3μs = 3000:1
The second method of dimming control uses the CTL pin
to linearly adjust the current sense threshold during the
PWM high state. When the CTL pin voltage is less than
1V, the LED current is:
I LED = V CTL
When V CTL is higher than 1.1V, the LED current is clamped
to 1A.
V IN
V CC
LTM8042
LED +
The LED current programming feature can be used in
conjunction with the PWM to possibly increase the total
BSTIN/BKLED –
RUN
TG
dimming range by an additional factor of ten.
TGEN
PWM
SYNC
SS
BSTOUT/BKIN
RT
GND
CTL
80421 TA01a
Figure 1. The LTM8042/LTM8042-1
Can Control a P-Channel PWM Switch
80421fa
17
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