LT3689 LINER [Linear Technology], LT3689 Datasheet - Page 19

• Manufacturer Part Number: LT3689
• Description: 700mA Step-Down Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Most applications running at V
require a small phase lead capacitor, ranging from 2pF to
about 30pF , between the FB pin and V
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 effi ciency 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
Figure 4. Transient Load Response of the LT3689 Front
Page Application as the Load Current is Stepped from
360mA to 720mA. V
250mA/DIV
50mV/DIV
V
OUT
I
L
OUT
= 3.3V, V
10μs/DIV
IN
IN
greater than 20V will
= 12V
OUT
for good transient
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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, V
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 effi ciency even at very
low loads (see Figure 5).
At higher output loads (above approximately 60mA at
V
running at the frequency programmed by the R
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 benefi t 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 12V
IN
= 12V for the front page application) the LT3689 will be
10mV/DIV
0.2A/DIV
5V/DIV
V
V
OUT
SW
IN
I
L
Figure 5. Burst Mode Operation
V
I
LOAD
and OUT quiescent currents are reduced
IN
= 12V, FRONT PAGE APPLICATION
= 8mA
IN
.
LT3689/LT3689-5
5μs/DIV
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T
resistor,
19
3689fa