Datasheet

LT8640/LT8640-1
16
Rev.C
For more information www.analog.com
APPLICATIONS INFORMATION
While in Burst Mode operation the current limit of the top
switch is approximately 900mA (as shown in Figure3),
resulting in low output voltage ripple. Increasing the output
capacitance will decrease output ripple proportionally. As
load ramps upward from zero the switching frequency
will increase but only up to the switching frequency
programmed by the resistor at the RT pin as shown in
Figure2a.
The output load at which the LT8640/LT8640-1 reaches
the programmed frequency varies based on input voltage,
output voltage and inductor choice. To select low ripple
Burst Mode operation, tie the SYNC/MODE pin below 0.4V
(this can be ground or a logic low output).
Forced Continuous Mode (LT8640-1 Only)
The LT8640-1 can operate in forced continuous mode
(FCM) for fast transient response and full frequency opera
-
tion over a wide load range. When in FCM, the oscillator
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed
at light loads or under large transient conditions. The
LT8640-1 can sink current from the output and return
this charge to the input in this mode, improving load step
transient response (see Figure4). At light loads, FCM
operation is less efficient than Burst Mode operation or
pulse-skipping mode, but may be desirable in applications
where it is necessary to keep switching harmonics out
of the signal band. FCM must be used if the output is
required to sink current. To enable FCM (LT8640-1 only),
float the SYNC/MODE pin. Leakage current on this pin
should be <1µA. See Block Diagram for internal pull-up
and pull-down resistance.
FCM is disabled if the V
IN
pin is held above 37V or if the FB
pin is held greater than 8% above the feedback reference
voltage. FCM is also disabled during soft-start until the
soft-start capacitor is fully charged. When FCM is disabled
in these ways, negative inductor current is not allowed and
the LT8640-1 operates in pulse-skipping mode.
For robust operation over a wide V
IN
and V
OUT
range, use
an inductor value greater than L
MIN
:
L
MIN
=
V
OUT
2 • f
SW
• 1–
V
OUT
40
⎛
⎝
⎜
⎞
⎠
⎟
FCM
BURST MODE
FRONT PAGE APPLICATION
100mA to 1.1A TRANSIENT
12V
IN
, 5V
OUT
C
OUT
= 47µF
50µs/DIV
V
OUT
200mA/DIV
I
LOAD
1A/DIV
8640 F04
Figure4. Load Step Transient Response with
and without Forced Continuous Mode
Figure3. Burst Mode Operation
I
L
500mA/DIV
V
SW
5V/DIV
5µs/DIV
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 10mA
V
SYNC
= 0V
8640 F03
Pulse-Skipping Mode (LT8640 Only)
For some applications, it is desirable for the LT8640 to
operate in pulse-skipping mode, offering two major differ
-
ences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. In this mode, much of the internal circuitry is
awake at all times, increasing quiescent current to several
hundred µA. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation
(see Figure2b). To enable pulse-skipping mode (LT8640
only), float the SYNC/MODE pin. Leakage current on this
pin should be <1µA. See Block Diagram for internal pull-up
and pull-down resistance.
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