LT3755-1 LINER [Linear Technology], LT3755-1 Datasheet - Page 19

• Manufacturer Part Number: LT3755-1
• Description: 60V 4-Switch Synchronous
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
For loop stability a lowpass RC filter is needed. For
most applications, a 50Ω resistor and 470nF capacitor
is sufficient.
Table 3
IVINMON
The IVINMON pin provides a linear indication of the current
flowing through the input. The equation for V
V
input, however, the output impedance of this pin is 12.5kΩ
so care must be taken not to load this pin.
Programming Output Voltage (Constant Voltage
Regulation) or Open LED/Overvoltage Threshold
For a voltage regulator, the output voltage can be set by
selecting the values of R5 and R6 (see Figure 9) according
to the following equation:
For an LED driver application, set the resistor from the
output to the FB pin such that the expected V
normal operation does not exceed 1.1V. Once V
than its overvoltage threshold, 1.25V (typical), the LT3791
stops switching.
(IVINP-IVINN)
V
OUT
= 1.2 •
R
IN
20
15
12
10
(mΩ)
6
5
4
3
2
• 20. This pin is suitable for driving an ADC
R5+R6
R6
I
LIMIT
10.0
12.5
16.7
2.5
3.3
4.2
5.0
8.3
25
(A)
FB
IVINMON
FB
is higher
during
is
Dimming Control
There are two methods to control the current source for
dimming using the LT3791. One method uses the CTRL
pin to adjust the current regulated in the LEDs. A second
method uses the PWM pin to modulate the current source
between zero and full current to achieve a precisely pro-
grammed average current. To make PWM dimming more
accurate, the switch demand current is stored on the VC
node during the quiescent phase when PWM is low. This
feature minimizes recovery time when the PWM signal goes
high. To further improve the recovery time a disconnect
switch may be used in the LED current path to prevent the
ISP node from discharging during the PWM signal low
phase. The minimum PWM on- or off-time is affected by
choice of operating frequency and external component
selection. The best overall combination of PWM and
analog dimming capabilities is available if the minimum
PWM pulse is at least six switching cycles and the PWM
pulse is synchronized to the SYNC signal.
SHORTLED Pin
The LT3791 provides an open-drain status pin,
SHORTLED, which pulls low when the FB pin is below
400mV. The only time the FB pin will be below 400mV
is during start-up or if the LEDs are shorted. During
Figure 9. Resistor Connection for Open LED Threshold
and Constant Output Voltage Regulation
LT3791
FB
V
OUT
R5
R6
3791 F09
LT3791
19
3791f