LT3755-1 LINER [Linear Technology], LT3755-1 Datasheet - Page 18

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

LT3791
APPLICATIONS INFORMATION
Programming LED Current
The LED current is programmed by placing an appropriate
value current sense resistor, R
string. The voltage drop across R
by the ISP and ISN pins. The CTRL pin should be tied to
a voltage higher than 1.2V to get the full-scale 100mV
(typical) threshold across the sense resistor. The CTRL
pin can also be used to dim the LED current, although
relative accuracy decreases with the decreasing sense
threshold. When the CTRL pin voltage is less than 1V,
the LED current is:
When the CTRL pin voltage is between 1.1V and 1.3V
the LED current varies with V
equation above by an increasing amount as V
increases. Ultimately, when V
no longer varies. The typical V
is listed in Table 2.
Table 2. V
When V
regulated to:
18
I
I
LED
LED
=
=
CTRL
(ISP-ISN)
100mV
V
V
R
CTRL
CTRL
LED
1.15
1.25
R
1.1
1.2
1.3
is higher than 1.3V, the LED current is
LED
(V)
Threshold vs CTRL
– 200mV
• 10
CTRL
(ISP-ISN)
CTRL
LED
, in series with the LED
> 1.3V the LED current
LED
, but departs from the
V
(ISP-ISN)
threshold vs V
is (Kelvin) sensed
94.5
99.5
100
90
98
(mV)
CTRL
voltage
CTRL
The CTRL pin should not be left open (tie to V
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for
the LED load, or with a resistor divider to V
output power and switching current when V
The presence of a time varying differential voltage signal
(ripple) across ISP and ISN at the switching frequency
is expected. The amplitude of this signal is increased by
high LED load current, low switching frequency and/or a
smaller value output filter capacitor. Some level of ripple
signal is acceptable: the compensation capacitor on the
VC pin filters the signal so the average difference between
ISP and ISN is regulated to the user-programmed value.
Ripple voltage amplitude (peak-to-peak) in excess of
20mV should not cause mis-operation, but may lead to
noticeable offset between the average value and the user-
programmed value.
ISMON
The ISMON pin provides a linear indication of the cur-
rent flowing through the LEDs. The equation for V
is V
input, however, the output impedance of this pin is 12.5kΩ
so care must be taken not to load this pin.
Programming Input Current Limit
The LT3791 has a standalone current sense amplifier. It
can be used to limit the input current. The input current
limit is calculated by the following equation:
I
IN
(ISP–ISN)
=
50mV
R
IN
• 10. This pin is suitable for driving an ADC
IN
IN
to reduce
REF
is low.
ISMON
if not
3791f