LTC3306

12

Rev. 0

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Enable Threshold Programming

The LTC3306 has a precision threshold enable pin to enable

or disable the switching. When forced low, the enable pin

puts the device into a low current shutdown mode.

The rising threshold of the EN comparator is 400mV, with

50mV of hysteresis. The EN pin can be tied to VIN if the

shutdown feature is not used. Adding a resistor divider

from VIN to EN programs the LTC3306 to regulate the

output only when VIN is above a desired voltage (see the

Block Diagram). Typically, this threshold, VIN(EN), is used

in situations where the input supply is current limited,

or has a relatively high source impedance. The VIN(EN)

threshold prevents the regulator from operating at source

voltages where low input voltage problems may occur.

This threshold can be adjusted by setting the values R1

and R2 such that they satisfy Equation 6.

VIN EN

( ) =

R1

R2

+ 1

⎛

⎝⎜

⎞

⎠⎟

• 400mV

(6)

The LTC3306 will remain in shutdown until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching will

not stop until the input falls 12.5% below VIN(EN).

FB Resistor Network (LTC3306 Adjustable Version)

The output voltage of the LTC3306 adjustable version is

programmed with a resistor divider between the output

and the FB pin. Choose the resistor values according to

Equation 7.

RA = RB

VOUT

500mV

– 1

⎛

⎝⎜

⎞

⎠⎟

(7)

as shown in Figure 2:

COUT

CFF

RA

SW

FB

3306 F02

BUCK

SWITCHING

REGULATOR

(OPTIONAL)

RB

L

VOUT

Figure 2. Feedback Resistor Network for the

LTC3306 Adjustable Version

1% or 0.1% resistors are recommended to maintain out-

put voltage accuracy. When optimizing the control loop

for high bandwidth and optimal transient response, add a

phase-lead capacitor connected from VOUT to FB.

APPLICATIONS INFORMATION