LTC3458_15 Datasheet, PDF (10/16 Pages) Linear Technology – 1.4A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3458
APPLICATIO S I FOR ATIO
transition occurs using a single resistor. During Burst
Mode operation, the oscillator is shut down, 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 deter-
mined 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
15μA of quiescent current. In this mode the output ripple
voltage has a variable frequency component with load
current and will be typically 2% peak-to-peak. This maxi-
mizes efficiency at very light loads by minimizing switch-
ing and quiescent losses. Burst Mode ripple can be re-
duced slightly by using more output capacitance (22μF or
greater). This capacitor does not need to be a low ESR type
if low ESR ceramics are also used. 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, COMP 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 minimum and maximum voltage on COMP during
Burst Mode operation. This minimizes the transient expe-
rienced 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 the BURST pin from
causing the part to oscillate in and out of Burst Mode
operation at the current where the mode transition occurs.
R_BURST
=
10
I_BURST
where R_BURST is in kΩ and I_BURST is in amps.
10
C _BURST
=
C_OUT • V_OUT
10,000
where C_BURST(MIN) and C_OUT are in μF.
Note: the BURST pin only sources current based on
current delivered to V_OUT through the P-channel MOSFET.
If current in the inductor is allowed to go negative (this can
occur at very light loads and high step-up ratios), the burst
threshold may become inaccurate, preventing the IC from
entering Burst Mode operation. For R_BURST values greater
than 200k, a larger than recommended inductor value may
be needed to ensure positive inductor current and auto-
matic Burst Mode operation.
In the event that a sudden load transient causes the voltage
level on FB 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 op-
eration and PWM mode, Burst can be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
drop. Note that Burst Mode operation is inhibited during
start-up and soft-start.
Manual Control
For applications requiring fixed frequency operation at all
load currents, connect the BURST pin to V_OUT through a
50kΩ resistor. To force Burst Mode operation, ground the
BURST pin.
For applications where a large load step can be anticipated,
the circuit below can be used to reduce the voltage
transient on V_OUT. Automatic operation is achieved when
the external PMOS is off and fixed frequency operation is
commanded when the external PMOS is on. In shutdown,
the PMOS should be off.
HIGH: AUTO MODE
LOW: FIXED FREQUENCY
V_IN
PMOS
BURST
133k
0.01μF
Figure 1
3458 FO2
3458fa