LTC3425 Datasheet, PDF (16/24 Pages) Linear Technology – 5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter

LTC3425
U
OPERATIO
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.
I_BURST
=
2.75
R_BURST
to leave Burst Mode operation
I_BURST
=
1.7
R_BURST
to enter Burst Mode operation
where R_BURST is in kΩ and I_BURST is in Amps. For load
currents under 20mA, refer to the curve Automatic Burst
Mode Thresholds vs R_BURST.
C_BURST
=
C_OUT • V_OUT
10,000
where C_BURST(MIN) and C_OUT are in µF.
When the voltage on BURST drops below 0.94V, the part
will enter Burst Mode operation. When the BURST pin
voltage is above 1.06V, it will be in fixed frequency mode.
In the event that a sudden load transient causes the
feedback pin to drop 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
transient 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. The circuit connected to BURST should be able
to sink up to 2mA. Note that Burst Mode operation is
inhibited during start-up and soft-start.
Note that if V_IN is raised to within 200mV or less below
V_OUT, the part will exit Burst Mode operation and the
synchronous rectifier will be disabled. It will remain in
fixed frequency mode until V_IN is at least 300mV below
V_OUT.
If the load applied during forced Burst Mode operation
(BURST = GND) exceeds the current that can be supplied,
the output voltage will start to droop and the part will
automatically come out of Burst Mode operation and enter
fixed frequency mode, raising V_OUT. The part will then
enter Burst Mode operation once again, the cycle will
repeat, resulting in about 4% output ripple. The maximum
current that can be supplied in Burst Mode operation is
given by:
I_O(MAX)
=
2
•


1+
0.60
V_OUT –
V_IN
V_IN


in Amps
Output Disconnect and Inrush Limiting
The LTC3425 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
PMOS rectifiers. This allows V_OUT to go to zero volts
during shutdown, drawing no current from the input
source. It also allows for inrush current limiting at turn-on,
minimizing surge currents seen by the input supply. Note
that to obtain the advantages of output disconnect, there
cannot be any external Schottky diodes connected be-
tween the switch pins and V_OUT.
Note: Board layout is extremely critical to minimize
voltage overshoot on the switch pins due to stray induc-
tance. Keep the output filter capacitors as close as
possible to the V_OUT pins, and use very low ESR/ESL
ceramic capacitors tied to a good ground plane.
For applications with V_OUT over 4.3V, Schottky diodes are
required to limit the peak switch voltage to less than 6V.
These must also be very close to minimize stray induc-
tance. See the section Applications Where V_OUT > 4.3V.
3425f
16