LT3668 datasheet(18/28 Pages) LINER

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LT3668

3668fa

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APPLICATIONS INFORMATION

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, the switching regulator will tolerate a shorted

output. There is another situation to consider in systems

where the output will be held high when the input to the

LT3668 is absent. This may occur in battery charging

applications or in battery backup systems where a battery

or some other supply is diode ORed with the switching

regulator’s output. If the IN1 pin is allowed to float and

the EN pin is held high (either by a logic signal or because

it is tied to IN1), then the LT3668’s internal circuitry will

pull its quiescent current through the SW pin. This is fine

if the system can tolerate a few μA in this state. If the EN

pin is grounded, the SW pin current will drop to 0.7μA.

However, if the IN1 pin is grounded while the output is

held high, regardless of EN, parasitic diodes inside the

LT3668 can pull current from the output through the SW

pin and the IN1 pin. Figure 4 shows a circuit that will run

only when the input voltage is present and that protects

against a shorted or reversed input.

LDOs

Adjustment Inputs

Each LDO output voltage of the LT3668 follows the voltage

at the corresponding adjustment pin ADJ2/ADJ3. Each

adjustmentpinispulleddownbyaninternalcurrentsource

(typically 200nA at 25°C). This current must be taken into

consideration if an adjustment pin is to be driven by a high

impedance resistive divider.

EvenifthevoltageatADJ2/ADJ3isbelowtheminimuminput

voltage, the corresponding output will always be regulated

to a voltage equal or below the voltage at ADJ2/ADJ3.

Anynoisepresentatanadjustmentpinistransferredtothe

corresponding output, in particular low frequency noise.

See the LDO transfer function in the Typical Performance

Characteristics section. Reference voltage noise can be

reduced by connecting a capacitor from ADJ2/ADJ3 to

ground. However, if the reference voltage is derived from

the resistive divider of the switching regulator as shown

in the application on page 1, no such bypass capacitor

is allowed as it would impair the switching regulator's

stability.

Figure 4. Diode D1 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output. It Also Protects the Circuit

from a Reversed Input, in Which Case the Resistor at the EN Pin

Limits the Current Drawn from That Pin. The LT3668 Runs Only

When the Input Is Present

IN3/BD

LT3668

BOOST

IN1

VIN

VOUT

BACKUP

3668 F04

MBRS140

FB1

GND

LT3668 Datasheet(PDF) 18 Page - Linear Technology

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