LTC3422
9
3422fa
OPERATIO
U
C
SW
= SW Capacitance) is low energy, but can cause EMI
radiation.
Burst Mode OPERATION
Burst Mode operation can be automatic or user controlled.
In automatic operation, the LTC3422 will automatically
enter Burst Mode operation at light load and return to fixed
frequency PWM mode for heavier loads. The user can
program the average load current at which the mode
transition occurs using a single resistor connected from
BURST to GND.
The oscillator is shut down during Burst Mode operation,
since the on time is determined by the time it takes the
inductor current to reach a fixed 600mA peak current and
the off time is determined by the time it takes for the
inductor current to return to zero.
In Burst Mode operation, the LTC3422 delivers energy to
the output until it is regulated and then enters a sleep state,
where the switches are kept off while the LTC3422 con-
sumes only 25µA of quiescent current. In this mode the
output ripple has a variable frequency component with
load current and will be typically 2% peak-peak. This
maximizes efficiency at very light loads by minimizing
switching and quiescent losses. Burst Mode operation
ripple can be reduced slightly by increasing the output
capacitance (47µF or greater). This additional capacitance
does not need to be a low ESR type if low ESR ceramics are
also used. Another method of reducing Burst Mode opera-
tion ripple is to place a small feed-forward capacitor (10pF
to 100pF) across the upper resistor in the V
OUT
feedback
divider network.
In Burst Mode operation, the compensation network is not
used and V
C
is disconnected from the error amplifier.
During long periods of Burst Mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fixed frequency mode of operation, even
at the same load current. To prevent this, the LTC3422
incorporates an active clamp circuit that holds the voltage
on V
C
at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fixed frequency mode operation.
Automatic Burst Mode Operation Control
For automatic operation, an RC network should be con-
nected from BURST to ground. The value of the resistor
will control the average load current (I
BURST
) at which
Burst Mode operation will be entered and exited (there is
hysteresis to prevent oscillation between modes). The
equation given for the capacitor on BURST is the minimum
value to prevent ripple on BURST from causing the part to
oscillate in and out of Burst Mode operation at the current
where the mode transition occurs. The equation given for
the resistor on BURST is the typical average load current
at which automatic Burst Mode operation is exited.
R
I
B
EXITBURST
=
12
where R
B
is in kΩ and I
EXITBURST
is in amps.
C
CV
B
OUT OUT
≥
•
,64 000
where C
B(MIN)
and C
OUT
are in µF.
Please refer to the Burst Mode Output Current Threshold
vs R
BURST
Typical Performance Chacteristic curves.
In the event that a load transient causes FB to drop by more
than 4% from the regulation value while in Burst Mode
operation, the LTC3422 will immediately switch to fixed
frequency operation and an internal pull-up will be mo-
mentarily applied to BURST, rapidly charging the BURST
capacitor. This prevents the LTC3422 from immediately
re-entering Burst Mode operation once the output achieves
regulation.
Manual Burst Mode Operation
For optimum transient response with large dynamic loads,
the operating mode should be controlled manually by the
host. By commanding fixed frequency PWM operation
prior to a sudden increase in load, output voltage droop
can be minimized. For manual control of Burst Mode
operation, the RC network connected to BURST can be
eliminated. To force fixed frequency PWM mode, BURST
should be connected to V
OUT
. To force Burst Mode opera-
tion, BURST should be grounded. When commanding
Burst Mode operation manually, the circuit connected to
