Applications Information - Analog Devices LINEAR LTM 8005 Manual

APPLICATIONS INFORMATION

Programming the Turn-On and Turn-Off Thresholds
with the EN/UVLO Pin
The falling under-voltage lockout (UVLO) value can be
accurately set by the resistor divider, as shown in Figure 1.
A small 3µA pull-down current is active when EN/UVLO
is below the threshold. The purpose of this current is to
allow the user to program the rising hysteresis. The fol-
lowing equations should be used to determine the values
of the resistors:
R1  499k + R2
V = 1.22 •
IN(FALLING)

V = V
IN(RISING) IN(FALLING)
LTM8005
499k
EN/UVLO
Figure 1. Set an Accurate UVLO with a Resistor Divider
Programming the Overvoltage Lockout Threshold with
the OVLO Pin
The input overvoltage lockout protection feature can be
implemented by a resistor from the V
shown in Figure 2. The following equations should be
used to determine the values of the resistors:
R3 + R4
V = 1.25 •
IN,OVLO
R4
LTM8005
Figure 2. Set an Overvoltage Lockout Threshold with
a Resistive Divider
R2
+ (3µA • R1 ! 499k)
V
IN
V
IN
R1
R2
8005 F01
to OVLO pins as
IN
V
IN
R3
OVLO
R4
8005 F02
For more information
LED Current Adjustment
The maximum output LED current is internally set to 1.6A,
typical. If both CTRL pins are tied to a voltage higher than
1.2V, maximum current is available. If a voltage less than
1.2V is applied to either CTRL1 or CTRL2, the LED current
will decrease. The two CTRL pins have identical func-
tions. Whichever is the lowest takes precedence. Either
CTRL pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
applied voltage sense threshold.
The CTRL pins should not be left open (tie to V
used). Either 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
Internal Power Switch Voltage Stress
The LTM8005 is equipped with an integrated ground
referred N-channel power MOSFET whose drain is con-
nected to the SW bank. The absolute maximum rating
of the SW bank is 40V. When using the LTM8005 in a
boost power topology, the voltage stress on the SW bank
is nominally a diode drop above V
buck-boost topologies, however, the voltage stress on the
SW bank is substantially higher than V
+ V . Do not exceed the absolute maximum voltage of
OUT
the SW bank under any operating condition.
Programming Output Voltage (Constant-Voltage
Regulation) and Output Voltage Open LED and
Shorted LED Thresholds
The LTM8005 has a voltage feedback pin FB that can be
used to program a constant-voltage output. In addition,
FB programming determines the output voltage that will
cause OPENLED and SHORTLED to assert. For a boost
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LTM8005
if not
REF
to reduce
IN
is low.
IN
. In the SEPIC or
OUT
, nominally V
OUT IN
Rev. B
11