Voltage Lockouts; Junction Temperature Measurement - Linear Technology Analog Devices LT8708 Datasheet

LT8708
APPLICATIONS INFORMATION
regulator is INTV . Therefore INTV
CC
voltage, typically > 4.0V, to properly regulate LDO33. The
LDO33 and INTV regulators are enabled by the SHDN
CC
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
APPROXIMATE
VOLTAGE
PIN(S) CONDITION
V <2.5V
INCHIP
SHDN <1.18V
INTV and
CC
<4.65V
GATEV
CC
SWEN <1.18V
LDO33 <3.04V
VINHIMON >1.207V
VOUTLOMON <1.207V
RVSOFF <1.209V
FBIN <1.205V
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 , SHDN and GND as shown in Figure 1. From
IN
the Electrical Characteristics, SHDN has typical rising and
46
must have sufficient
CC
CHIP STATE
(Figure 2) READ SECTION
CHIP OFF
Operation: Start-Up
SWITCHER
OFF 1
Applications
Information:
VINHIMON,
–
VOUTLOMON and
RVSOFF
Voltage Lockouts
For more information
falling thresholds of 1.221V and 1.181V, respectively. The
falling threshold for turning-off switching activity can be
chosen using:
R
=
SHDN1
R • (V
SHDN2 (IN,CHIPOFF,FALLING)
1.181
For example, choosing R
threshold of 5.42V results in:
R
=
SHDN1
20k • (5.42 – 1.181)
1.181
The rising threshold for enabling switching activity
would be:
V = V
(IN,CHIPOFF,RISING)
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 low will stop all switching activity. Note that this does
C
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
of the CLKOUT signal and use the following equation to
approximate the junction temperature:
DC – 34.4%
CLKOUT
T
≅
J
0.325%
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– 1.181)
Ω
= 20k and a falling V
SHDN2
≅ 71.5kΩ
1.221
•
(IN,CHIPOFF,FALLING)
1.181
: Regulation
IN
. Measure the duty cycle
J
°C
IN
Rev 0