Datasheet
LTC3418
11
3418fb
Burst Clamp Programming
If the voltage on the SYNC/MODE pin is less than V
IN
by
1V, Burst Mode operation is enabled. During Burst Mode
operation, the voltage on the SYNC/MODE pin determines
the burst clamp level, which sets the minimum peak induc-
tor current, I
BURST
, for each switching cycle. A graph show-
ing the relationship between the minimum peak inductor
current and the voltage on the SYNC/MODE pin can be
found in the Typical Performance Characteristics section.
In the graph, V
BURST
is the voltage on the SYNC/MODE
pin. I
BURST
can only be programmed in the range of 0A
to 10A. For values of V
BURST
less than 0.4V, I
BURST
is set
at 0A. As the output load current drops, the peak inductor
currents decrease to keep the output voltage in regulation.
When the output load current demands a peak inductor
current that is less than I
BURST
, the burst clamp will force
the peak inductor current to remain equal to I
BURST
regard-
less of further reductions in the load current. Since the
average inductor current is greater than the output load
current, the voltage on the I
TH
pin will decrease. When
the I
TH
voltage drops to 350mV, sleep mode is enabled
in which both power MOSFETs are shut off and switching
action is discontinued to minimize power consumption.
All circuitry is turned back on and the power MOSFETs
begin switching again when the output voltage drops out
of regulation. The value for I
BURST
is determined by the
desired amount of output voltage ripple. As the value of
I
BURST
increases, the sleep period between pulses and the
output voltage ripple increase. The burst clamp voltage,
V
BURST
, can be set by a resistor divider from the V
FB
pin
to the SGND pin as shown in the Typical Application on
the front page of this data sheet.
Pulse skipping, which is a compromise between low output
voltage ripple and effi ciency during low load current opera-
tion, can be implemented by connecting the SYNC/MODE
V
FB
V
OUT
R1
3418 F01
R2
SGND
LTC3418
Figure 1. Setting the Output Voltage
pin to ground. This sets I
BURST
to 0A. In this condition, the
peak inductor current is limited by the minimum on-time
of the current comparator; and the lowest output voltage
ripple is achieved while still operating discontinuously.
During very light output loads, pulse skipping allows only
a few switching cycles to be skipped while maintaining
the output voltage in regulation.
Voltage Tracking
The LTC3418 allows the user to program how its output
voltage ramps during start-up by means of the TRACK
pin. Through this pin, the output voltage can be set up to
either track coincidentally or ratiometrically follow another
output voltage as shown in Figure 2. If the voltage on the
TRACK pin is less than 0.8V, voltage tracking is enabled.
During voltage tracking, the output voltage regulates to
the tracking voltage through a resistor divider network.
V
OUT2
V
OUT1
3418 F02a
TIME
OUTPUT VOLTAGE
V
OUT2
V
OUT1
3418 F02a
TIME
OUTPUT VOLTAGE
Figure 2a. Coincident Tracking
Figure 2b. Ratiometric Sequencing
APPLICATIONS INFORMATION