LT8708
46
Rev 0
For more information
regulator is INTV
CC
. Therefore INTV
CC
must have sufficient
voltage, typically > 4.0V, to properly regulate LDO33. The
LDO33 and INTV
CC
regulators are enabled by the
SHDN
pin and are not affected by SWEN. The LDO33 pin regu-
lator has overcurrent protection circuitry that typically
limits the output current to 17.25mA. An undervoltage
lockout monitors LDO33 and disables switching activity
when LDO33 falls below 3.04V (typical). LDO33 should
be bypassed locally with 0.1μF or more.
VOLTAGE LOCKOUTS
The LT8708 contains several voltage detectors to make
sure the chip is under proper operating conditions. Table
8 summarizes the pins that are monitored and also indi-
cates the state that the LT8708 will enter if an under or
over voltage condition is detected.
Table 8. Voltage Lockout Conditions
PIN(S)
APPROXIMATE
VOLTAGE
CONDITION
CHIP STATE
READ SECTION
V
INCHIP
<2.5V
CHIP OFF
Operation: Start-Up
SHDN
<1.18V
INTV
CC
and
GATEV
CC
<4.65V
SWITCHER
OFF 1
SWEN
<1.18V
LDO33
<3.04V
VINHIMON
>1.207V
–
Applications
Information:
VINHIMON,
VOUTLOMON and
RVSOFF
VOUTLOMON
<1.207V
RVSOFF
<1.209V
FBIN
<1.205V
Voltage Lockouts
The conditions are listed in order of priority from top
to bottom. If multiple over/undervoltage conditions are
detected, the chip will enter the state listed highest on
the table.
Due to their accurate thresholds, configurable undervoltage
lockouts (UVLOs) can be implemented using the
SHDN
and
SWEN and in some cases, FBIN pin. The UVLO function
sets the turn on/off of the LT8708 at a desired minimum
voltage. For example, a resistor divider can be connected
between V
IN
,
SHDN
and GND as shown in Figure 1. From
the Electrical Characteristics,
SHDN
has typical rising and
falling thresholds of 1.221V and 1.181V, respectively. The
falling threshold for turning-off switching activity can be
chosen using:
R
SHDN1
=
R
SHDN2
•(V
(IN,CHIPOFF,FALLING)
– 1.181)
1.181
Ω
For example, choosing R
SHDN2
= 20k and a falling V
IN
threshold of 5.42V results in:
R
SHDN1
=
20k •(5.42 – 1.181)
1.181
≅
71.5kΩ
The rising threshold for enabling switching activity
would be:
V
(IN,CHIPOFF,RISING)
=
V
(IN,CHIPOFF,FALLING)
•
1.221
1.181
or 5.6V in this example.
Similar calculations can be used to select a resistor divider
connected to SWEN that would stop switching activity dur-
ing an undervoltage condition. Make sure that the divider
doesn’t cause SWEN to exceed 7V (ABSMAX rating) under
maximum supply voltage conditions. See the Start-Up:
SWEN Pin section for additional information.
The same technique described in the V
section can be used to create an undervoltage lockout if
the LT8708 is in forward non-CCM mode, where forcing
V
C
low will stop all switching activity. Note that this does
not reset the soft-start function, therefore resumption of
switching activity will not be accompanied by a soft-start.
JUNCTION TEMPERATURE MEASUREMENT
The duty cycle of the CLKOUT signal is linearly proportional
to the die junction temperature, T
J
. Measure the duty cycle
of the CLKOUT signal and use the following equation to
approximate the junction temperature:
T
J
≅
DC
CLKOUT
– 34.4%
0.325%
°C
APPLICATIONS INFORMATION
