LTC3407AIDD Datasheet, PDF (7/16 Pages) Linear Integrated Systems – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407
OPERATION
decrease causes the error amplifier to increase the I_TH
voltage until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the RUN
pin to ground.
Low Current Operation
Two modes are available to control the operation of the
LTC3407 at low currents. Both modes automatically switch
from continuous operation to to the selected mode when
the load current is low.
To optimize efficiency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3407
automatically switches into Burst Mode operation in which
the PMOS switch operates intermittently based on load
demand with a fixed peak inductor current. By running
cycles periodically, the switching losses which are domi-
nated by the gate charge losses of the power MOSFETs
are minimized. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
A hysteretic voltage comparator trips when I_TH is below
0.35V, shutting off the switch and reducing the power. The
output capacitor and the inductor supply the power to the
load until I_TH exceeds 0.65V, turning on the switch and the
main control loop which starts another cycle.
For lower ripple noise at low currents, the pulse-skipping
mode can be used. In this mode, the LTC3407 continues to
switch at a constant frequency down to very low currents,
where it will begin skipping pulses.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R_DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (See Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3407 is used at 100% duty cycle with low
input voltage (See Thermal Considerations in the Applica-
tions Information Section).
Low Supply Operation
The LTC3407 incorporates an Under-Voltage Lockout circuit
which shuts down the part when the input voltage drops
below about 1.65V to prevent unstable operation.
APPLICATIONS INFORMATION
A general LTC3407 application circuit is shown in Figure 2.
External component selection is driven by the load require-
ment, and begins with the selection of the inductor L. Once
the inductor is chosen, C_IN and C_OUT can be selected.
Inductor Selection
Although the inductor does not influence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ΔI_L decreases
with higher inductance and increases with higher V_IN or
V_OUT:
ΔI_L
=
V_OUT
f_O • L
⎛
• ⎜1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
Accepting larger values of ΔI_L allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability. A
reasonable starting point for setting ripple current is ΔI_L =
0.3 • I_LIM, where I_LIM is the peak switch current limit. The
largest ripple current ΔI_L occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specified maximum, the inductor value should be chosen
according to the following equation:
L
≥
V_OUT
f_O • ΔI_L
⎛
• _{⎝⎜⎜}1–
V_OUT
V_{IN(MAX )}
⎞
⎠⎟⎟
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
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