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LT1579

if VIN2 >VIN1, so typically only 3µA. The primary input will

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

µA.

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

µAdrawnfromtheprimary,2µAdrawnfromthe

secondary and 3

µA 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

µA. 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).

APPLICATIONS INFORMATION

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 (ISINK) when the comparator

is tripped low. At no load, comparator hysteresis is zero,

increasing to a maximum of 18mV for sink currents above

µA. 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 VLBO can be done using a load line on the curve

of Logic Flag Output Voltage vs Sink Current in the Typical

Performance Characteristics.

Figure 2. Connecting SS to Low-Battery Detector

Output to Prevent Damage to Batteries

LBO

GND

VCC

1579 F02

–

1.5V

VTRIP

VOUT

LBI

LBO

ISINK

LTC1579 • F02

R2 = (VTRIP – 1.5V)

1.5V

()

HYSTERESIS = VHYST 1 +

FOR ISINK ≥ 20µA, VHYST = 18mV,

FOR ISINK < 20µA, SEE THE TYPICAL

PERFORMANCE CHARACTERISTICS

()

R3 =

(1.5V + VHYST – 0.6V – VLBO)(R2)

VHYST(ADDED)

FOR ADDED HYSTERESIS

Figure 3. Low-Battery Comparator Operation

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