LT3743_10 Datasheet, PDF (16/28 Pages) Linear Technology – High Current Synchronous Step-Down LED Driver with Three-State Control

LT3743
APPLICATIONS INFORMATION
V_{CC_INT} Capacitor Selection
The bypass capacitor for the V_{CC_INT} pin should be larger
than 5μF for stability and has no ESR requirement. It
is recommended that the ESR be lower than 50mΩ to
reduce noise within the LT3743. For driving MOSFETs
with gate charges larger than 10nC, use 0.5μF/nC of total
gate charge.
LED Current Dimming
The LT3743 provides the capability of traditional zero to full
current PWM dimming as well as PWM dimming between
two regulated LED current states. When the PWM signal
is low, no switching occurs and the output capacitors
are disconnected from ground. When PWM is high and
CTRL_SEL is low, the inductor current is regulated to the
low current state. In this state, the PWMGL signal is high,
connecting the output capacitor for the low regulated
current state. When PWM and CTRL_SEL are both high,
the inductor current is regulated to the high current state.
In this state, the PWMGH signal is high, connecting the
output capacitor for the high regulated current state. The
transition time between each of the regulated inductor
currents is determined by the inductor size, V_IN and V_O.
Due to the use of the switched output capacitors, the LED
current will begin to flow within 130ns of the transition on
the CTRL_SEL pin. Figure 8 shows the LED and inductor
current waveforms with the various states of the control
signals.
To adjust the regulated LED current for the two control
states, an analog voltage is applied to the CTRL_L and
CTRL_H pins. Figure 6 shows the regulated voltage across
the sense resistor for control voltages up to 2V. Figure 7
shows the CTRL_L voltage created by a voltage divider
from V_REF to ground. When sizing the resistor divider,
please be aware that the V_REF pin is current limited to
500μA. Above 1.5V, the control voltage has no effect on
the regulated LED current.
For the widest dimming range, use the highest switching
frequency possible and lowest PWM frequency. For con-
figuration with the maximum PWM range, please contact
factory for optimized component selection.
60
50
40
30
20
10
0
0
0.5
1.0
1.5
2.0
V_CTRL (V)
3743 F06
Figure 6. LED Current vs CTRL Voltage
V_REF
LT3743
R2
CTRL_L
R1
3743 F07
Figure 7. Analog Control of LED Current
PWM
t_PWM
t_ON(PWM)
CTRL_SEL
INDUCTOR
CURRENT
PWMGH
PWMGL
LED
CURRENT
ICTRL_H
ICTRL_L
3743 F08
Figure 8. LED Current vs CTRL Voltage
3743fb
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