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LT3689/LT3689-5

3689fa

Synchronization

To select low ripple Burst Mode operation, tie the SYNC pin

below 0.3V (this can be ground or a logic output). Synchro-

nizing the LT3689 oscillator to an external frequency can

be done by connecting a square wave (with positive and

negative pulse width >80ns) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.3V

and peaks that are above 1V (up to 6V).

The LT3689 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will skip pulses to maintain regulation.

The LT3689 may be synchronized over a 350kHz to 2.5MHz

range. The RT resistor should be chosen to set the LT3689

switching frequency 20% below the lowest synchronization

input. For example, if the synchronization signal will be

350kHz and higher, the RT should be chosen for 280kHz.

To assure reliable and safe operation, the LT3689 will only

synchronize when the output voltage is above 90% of its

regulated voltage. It is therefore necessary to choose a

large enough inductor value to supply the required output

current at the frequency set by the RT resistor (see the

Inductor Selection section). It is also important to note

that the slope compensation is set by the RT value. When

the sync frequency is much higher than the one set by

RT, the slope compensation will be signiﬁcantly reduced,

which may require a larger inductor value to prevent

subharmonic oscillation. The minimum inductor value

should be calculated using the RT programmed frequency

to avoid subharmonic oscillation.

Shutdown and Undervoltage Lockout

Figure 9 shows how to add undervoltage lockout (UVLO)

to the LT3689. Typically, UVLO is used in situations where

the input supply is current limited, or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so source current increases as

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. UVLO

prevents the regulator from operating at source voltages

where the problems might occur. An internal comparator

will force the part into shutdown below the minimum VIN

of 3.4V. This feature can be used to prevent excessive

discharge of battery-operated systems. If an adjustable

UVLO threshold is required, the EN/UVLO pin can be used.

The threshold voltage of the EN/UVLO pin comparator is

1.26V. Current hysteresis is added above the EN threshold.

This can be used to set voltage hysteresis of the UVLO

using the following:

R3 =

V H VL

4µA

R4 =

1.26V

VH 1.26V

4µA

APPLICATIONS INFORMATION

Figure 9. Undervoltage Lockout

1.25V

4μA

EN/UVLO

LT3689

VIN

3689 F09

LT3689 Datasheet(PDF) 22 Page - Linear Technology

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