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LT3972

3972f

Synchronizing the LT3972 oscillator to an external fre-

quency can be done by connecting a square wave (with

20% to 80% duty cycle) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.3V

and peaks that are above 0.8V (up to 6V).

The LT3972 will not enter Burst Mode at low output loads

while synchronized to an external clock, but instead will

skip pulses to maintain regulation.

The LT3972 may be synchronized over a 250kHz to 2MHz

range. The RT resistor should be chosen to set the LT3972

switching frequency 20% below the lowest synchronization

input. For example, if the synchronization signal will be

250kHz and higher, the RT should be chosen for 200kHz.

To assure reliable and safe operation the LT3972 will only

synchronize when the output voltage is near regulation

as indicated by the PG ﬂag. 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

Inductor Selection section. It is also important to note that

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.

Shorted and Reversed Input Protection

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

sively, an LT3972 buck 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

LT3972 is absent. This may occur in battery charging ap-

plications or in battery backup systems where a battery

or some other supply is diode OR-ed with the LT3972’s

output. If the VIN pin is allowed to ﬂoat and the RUN/SS

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

tied to VIN), then the LT3972’s internal circuitry will pull

its quiescent current through its SW pin. This is ﬁne if

your system can tolerate a few mA in this state. If you

ground the RUN/SS pin, the SW pin current will drop to

essentially zero. However, if the VIN pin is grounded while

the output is held high, then parasitic diodes inside the

LT3972 can pull large currents from the output through

the SW pin and the VIN pin. Figure 8 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Figure 8. Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also

Protects the Circuit from a Reversed Input. The LT3972

Runs Only When the Input is Present

VIN

BOOST

GND FB

RUN/SS

MBRS340

VIN

LT3972

3680 F08

VOUT

BACKUP

APPLICATIONS INFORMATION

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 9 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents ﬂow in the LT3972’s VIN and SW pins, the catch

diode (D1) and the input capacitor (C1). The loop formed

by these components should be as small as possible. These

components, along with the inductor and output capacitor,

should be placed on the same side of the circuit board,

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components.

The SW and BOOST nodes should be as small as possible.

Finally, keep the FB and VC nodes small so that the ground

Figure 7. To Soft-Start the LT3972, Add a Resisitor

and Capacitor to the RUN/SS Pin

3680 F07

1A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

RUN/SS

GND

RUN

15k

2ms/DIV

0.22μF

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