Datasheet

LT3791-1
16
37911f
Figure 8. Resistor Connection to Set V
IN
UVLO and
OVLO Thresholds
LT3791-1
V
IN
R1
R3
R4
R2
37911 F08
EN/UVLO
OVLO
The rising OVLO value can be accurately set by the resis-
tor divider R3 and R4. The following equations should be
used to determine the resistor values:
V
IN(OVLO
+
)
= 3 •
R3+R4
R4
V
IN(OVLO
–
)
= 2.925 •
R3+R4
R4
applicaTions inForMaTion
Table 2. V
(ISP-ISN)
Threshold vs CTRL
V
CTRL
(V) V
(ISP-ISN)
(mV)
1.1 90
1.15 94.5
1.2 98
1.25 99.5
1.3 100
When V
CTRL
is higher than 1.3V, the output current is
regulated to:
I
OUT
=
100mV
R
OUT
The CTRL pin should not be left open (tie to V
REF
if not
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for
the output load, or with a resistor divider to V
IN
to reduce
output power and switching current when V
IN
is low.
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 output 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 V
C
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 current
flowing through the output. The equation for V
ISMON
is
V
(ISP–ISN)
• 10. This pin is suitable for driving an ADC 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
LT3791-1 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:
Programming Output Current
The output current is programmed by placing an appro-
priate value current sense resistor, R
OUT
, in series with
the output load. The voltage drop across R
OUT
is (Kelvin)
sensed 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 adjust the output current,
although relative accuracy decreases with the decreasing
sense threshold. When the CTRL pin voltage is less than
1V, the output current is:
I
OUT
=
V
CTRL
– 200mV
R
OUT
• 10
When the CTRL pin voltage is between 1.1V and 1.3V the
output current varies with V
CTRL
, but departs from the
equation above by an increasing amount as V
CTRL
volt-
age increases. Ultimately, when V
CTRL
> 1.3V the output
current no longer varies. The typical V
(ISP-ISN)
threshold
vs V
CTRL
is listed in Table 2.