LTC3421_15 Datasheet, PDF (9/16 Pages) Linear Technology – 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect

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OPERATIO
Zero Current Amplifier
The zero current amplifier monitors the inductor current to
the output and shuts off the synchronous rectifier once the
current is below 50mA typical, preventing negative induc-
tor current.
Antiringing Control
The antiringing control places a resistor across the
inductor to damp the ringing on the SW pin in discontinu-
ous conduction mode. The LC_SW ringing (L = inductor,
C_SW = capacitance on SW pin) is low energy, but can
cause EMI radiation.
V_REF
The internal 1.22V reference is buffered and brought out
to REFOUT. It is active when the ENB pin is pulled high
(above 1.4V). For stability, a minimum of a 0.1µF capacitor
must be placed on the pin. The output can source up to
100µA and sink up to 8µA. For the lowest possible quies-
cent current in Burst Mode operation, the reference output
should be disabled by grounding the ENB pin.
Burst Mode OPERATION
Burst Mode operation can be automatic or user controlled.
In automatic operation, the IC 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.
The oscillator is shut down in this mode, since the on time
is determined by the time it takes the inductor current to
reach a fixed 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 IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
12µA 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 ripple can be reduced slightly by
LTC3421
increasing the output capacitance. Another method of
reducing Burst Mode ripple is to place a small feed-
forward capacitor across the upper resistor in the V_OUT
feedback divider network.
During Burst Mode operation, the V_C pin is disconnected
from the error amplifier in an effort to hold the voltage on
the external compensation network where it was before
entering Burst Mode operation. To minimize the effects of
leakage current and stray resistance, voltage clamps limit
the min and max voltage on V_C during Burst Mode opera-
tion. This minimizes the transient experienced when a
heavy load is suddenly applied to the converter after being
in Burst Mode operation for an extended period of time.
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 for 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.
R_BURST
=
2
I_BURST
where R_BURST is in kΩ and I_BURST is in amps.
C_BURST
≥
C_OUT • V_OUT
10,000
where C_BURST(MIN) and C_OUT are in µF.
In the event that a sudden load transient causes FB to
deviate by more than 4% from the regulation value, an
internal pull-up is applied to BURST, forcing the part
quickly out of Burst Mode operation. For optimum tran-
sient response when going between Burst Mode operation
and PWM mode, the mode should be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
droop. For manual control of Burst Mode operation, the
RC network can be eliminated. To force fixed frequency
PWM mode, BURST should be connected to V_OUT. To
force Burst Mode operation, BURST should be grounded.
3421f
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