LT3689 datasheet(19/32 Pages) LINER

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LT3689/LT3689-5

3689fa

Most applications running at VIN greater than 20V will

require a small phase lead capacitor, ranging from 2pF to

about 30pF, between the FB pin and VOUT for good transient

response. The circuits in the Typical Applications section

use the appropriate phase lead capacitors and are stable

at all input voltages.

If the output capacitor is different than the recommended

capacitor, stability should be checked across all operating

conditions, including load current, input voltage and tem-

perature. The LT1375 data sheet contains a more thorough

discussion of loop compensation and describes how to

test the stability using a transient load. Figure 4 shows

the transient response when the load current is stepped

from 360mA to 720mA and back to 360mA.

Low Ripple Burst Mode Operation and Pulse-Skipping

Mode

The LT3689 is capable of operating in either low ripple Burst

Mode operation or pulse-skipping mode, which is selected

using the SYNC pin. See the Synchronization section for

details. To enhance efﬁciency at light loads, the LT3689

can be operated in low ripple Burst Mode operation that

keeps the output capacitor charged to the proper voltage

while minimizing the input quiescent current. During Burst

Mode operation, the LT3689 delivers single cycle bursts of

current to the output capacitor followed by sleep periods

where the output power is delivered to the load by the

output capacitor. Because the LT3689 delivers power to the

output with single, low current pulses, the output ripple is

kept below 15mV for a typical application. The LT3689-5

has a slightly higher output ripple of 25mV. This higher

ripple can be reduced by using a larger output capacitor.

In addition, VIN and OUT quiescent currents are reduced

to typically 50μA and 75μA, respectively, during the sleep

time. As the load current decreases towards a no-load

condition, the percentage of time that the LT3689 operates

in sleep mode increases and the average input current is

greatly reduced, resulting in high efﬁciency even at very

low loads (see Figure 5).

At higher output loads (above approximately 60mA at

VIN = 12V for the front page application) the LT3689 will be

running at the frequency programmed by the RT resistor,

and will be operating in standard PWM mode. The transi-

tion between PWM and low ripple Burst Mode operation

is seamless, and will not disturb the output voltage. If low

quiescent current is not required, the LT3689 can oper-

ate in pulse-skipping mode. The beneﬁt of this mode is

that the LT3689 will enter full frequency standard PWM

operation at a lower output load current than when in

Burst Mode operation. The front page application circuit

will switch at full frequency at output loads higher than

about 15mA at 12VIN.

APPLICATIONS INFORMATION

Figure 4. Transient Load Response of the LT3689 Front

Page Application as the Load Current is Stepped from

360mA to 720mA. VOUT = 3.3V, VIN = 12V

Figure 5. Burst Mode Operation

10μs/DIV

VOUT

50mV/DIV

250mA/DIV

3689 F04

5μs/DIV

VOUT

10mV/DIV

VIN = 12V, FRONT PAGE APPLICATION

ILOAD = 8mA

VSW

5V/DIV

0.2A/DIV

3689 F05

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