LT1579CS8-3.3 Linear Technology, LT1579CS8-3.3 Datasheet - Page 14

• Manufacturer Part Number: LT1579CS8-3.3
• Description: IC LDO REG SMART DUAL 3.3V 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1579CS8-3.3

• Function: Back-Up Management
• Battery Type: Smart Batteries
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
LT1579
if V
automatically go into standby mode as the primary input
drops below the output voltage. The primary input can also
be forced into standby mode by asserting the SS pin. In
either case, the current drawn from the primary input is
reduced to a maximum of 7 A.
Shutdown
The LT1579 has a low power shutdown state where all
functions of the device are shut off. The device is put into
shutdown mode when the shutdown pin is pulled below
0.7V. The quiescent current in shutdown has three com-
ponents: 2 A drawn from the primary, 2 A drawn from the
secondary and 3 A which is drawn from the higher of the
two inputs.
Protecting Batteries Using Secondary Select (SS)
Some batteries, such as lithium-ion cells, are sensitive to
deep discharge conditions. Discharging these batteries
below a certain threshold severely shortens battery life. To
prevent deep discharge of the primary cells, the LT1579
secondary select (SS) pin can be used to switch power
draw from the primary input to the secondary. When this
pin is pulled low, current out of the primary is reduced to
2 A. A low-battery detector with the trip point set at the
critical discharge point can signal the low battery condi-
tion and force the switchover to the secondary as shown
in Figure 2. The second low-battery comparator can be
used to set a latch to shutdown the LT1579 (see the Typical
Applications).
14
IN2
>V
Figure 2. Connecting SS to Low-Battery Detector
Output to Prevent Damage to Batteries
IN1
, so typically only 3 A. The primary input will
V
CC
U
R
P
INFORMATION
LBO
SS
U
GND
1579 F02
W
U
Low-Battery Comparators
There are two independent low-battery comparators in the
LT1579. This allows for individual monitoring of each
input. The inverting inputs of both comparators are con-
nected to an internal 1.5V reference. The low-battery
comparator trip point is set by an external resistor divider
as shown in Figure 3. The current in R1 at the trip point is
1.5V/R1. The current in R2 is equal to the current in R1.
The low-battery comparator input bias current, 2nA flow-
ing out of the pin, is negligible and may be ignored. The
value of R1 should be less than 1.5M in order to minimize
errors in the trip point. The value of R2 for a given trip point
is calculated using the formulas in Figure 3.
The low-battery comparators have a small amount of
hysteresis built-in. The amount of hysteresis is dependent
upon the output sink current (I
is tripped low. At no load, comparator hysteresis is zero,
increasing to a maximum of 18mV for sink currents above
20 A. See the curve of Low-Battery Comparator Hyster-
esis in the Typical Performance Characteristics. If larger
amounts of hysteresis are desired, R3 and D1 can be
added. D1 can be any small diode, typically a 1N4148.
Calculating V
of Logic Flag Output Voltage vs Sink Current in the Typical
Performance Characteristics.
Figure 3. Low-Battery Comparator Operation
LBO
V
TRIP
can be done using a load line on the curve
R2
R1
1.5V
R2 = (V
HYSTERESIS = V
R3 =
FOR I
FOR I
PERFORMANCE CHARACTERISTICS
FOR ADDED HYSTERESIS
R3
(1.5V + V
SINK
SINK
LBI
TRIP
< 20 A, SEE THE TYPICAL
+
–
20 A, V
– 1.5V)
HYST
V
HYST
HYST(ADDED)
SINK
D1
HYST
– 0.6V – V
1.5V
R1
1 +
) when the comparator
= 18mV,
R2
R1
LBO
I
LTC1579 • F02
SINK
LBO
V
)(R2)
OUT
R4