LTC3407-2_15 Datasheet, PDF (7/16 Pages) Linear Technology – Dual Synchronous, 800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3407-2
OPERATION
Low Current Operation
Two modes are available to control the operation of the
LTC3407-2 at low currents. Both modes automatically
switch from continuous operation 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 LTC3407-2
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 LTC3407-2 continues
to switch at a constant frequency down to very low cur-
rents, where it will begin skipping pulses. The efficiency in
pulse-skipping mode can be improved slightly by connect-
ing the SW node to the MODE/SYNC input which reduces
the clock frequency by approximately 30%.
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 LTC3407-2 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Low Supply Operation
To prevent unstable operation, the LTC3407-2 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 1.65V.
APPLICATIONS INFORMATION
A general LTC3407-2 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, 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 operating
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:

I_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 = 0.4 • I_OUT(MAX), where I_OUT(MAX) is 800mA. 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 • I_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
load currents. This causes a dip in efficiency in the upper
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