LTC3456 Datasheet, PDF (19/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

U
OPERATIO
VOLTAGE MONITORS
Low-Battery Detection
The LTC3456 has an on-chip gain block that can be used
for low-battery detection. The low-battery trip point can be
set by two resistors (Figure 10). The nominal voltage at
AIN is 0.8V. If the voltage at AIN falls below 0.8V, AO sinks
current to ground.
The battery minimum voltage can be set according to the
formula:
V_BATT(MIN) = 0.8V^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
The AIN input bias current is quite low, on the order of 2nA
(typ). Large resistor values (R2 ~ 100k) can be used in the
divider network. This helps in minimizing the loading on
the battery. AO is an open-drain logic output. The voltage
at AIN must always be kept less than 2V. If the gain block
is not used then connect AIN to ground.
There is no built-in hysteresis in the gain block. Hysteresis
can be added by connecting resistor R4 from AIN to AO as
shown in Figure 10. Ensure that R4 ≅ 10R3 for correct
operation. With the values shown in Figure 10b, the circuit
has 180mV of hysteresis.
V_BATT
1.8V TO 3.2V
V_CORE
R2
R3
100k LTC3456
100k
AIN AO
R1
80.6k
(10a)
V_BATT
R2
100k
R1
80.6k
V_CORE
R3
LTC3456
100k
AIN AO
R4
1M
3456 F10
(10b)
Figure 10. Low-Battery Detector (10a) and
Low-Battery Detector with Hysteresis (10b)
LTC3456
The gain block can be configured to drive an external PNP
transistor and generate an auxiliary voltage as shown in
Figure 11. An auxiliary output voltage 2.5V/20mA is gen-
erated from the V_MAIN (3.3V) power supply.
V_MAIN
3.3V
100k
Q1
LTC3456
AIN AO
PHILIPS
MMBT3906
V_AUX
2.5V
43.2k ^20mA
2.2µF
20.5k
3456 F11
Figure 11. Generating Auxiliary Voltage Supply
External Power Detection
The LTC3456 has an EXT_PWR output pin to indicate the
presence of USB or wall power. Whenever the WALLFB pin
is pulled higher than 1.25V (AC adapter present), or the
USB input is greater than 4V and the SUSPEND pin is low
(USB power available), the EXT_PWR pin is pulled to
ground. When pulled low, this pin is capable of sinking
5mA suitable for driving an external LED. Otherwise, this
pin is in a high impedance state.
Overtemperature Protection
The maximum allowable junction temperature for LTC3456
is 125°C. In normal operation, the IC does not dissipate
much heat and its junction temperature stays well below
125°C at an ambient temperature of 85°C or less. If the
junction temperature exceeds 150°C, the Core, Main and
Hot Swap outputs are turned off and RESET is pulled low.
The V_INT output stays alive in this state. The Core and Main
outputs will remain off until the die temperature falls below
150°C, regardless of the state of the PWRKEY and PWRON
inputs.
3456fa
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