LTC3548 Datasheet, PDF (7/16 Pages) Linear Technology – Dual Synchronous, 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

U
OPERATIO
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
By selecting MODE/SYNC (pin 6), two modes are available
to control the operation of the LTC3548 at low currents.
Both modes automatically switch from continuous opera-
tion 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 LTC3548
automatically switches into Burst Mode operation, in
which the PMOS switch operates intermittently based on
load demand with a fixed peak inductor current. By run-
ning cycles periodically, the switching losses which are
dominated 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
voltage comparator trips when I_TH is below 0.35V, shut-
ting 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
LTC3548
mode can be used. In this mode, the LTC3548 continues
to switch at a constant frequency down to very low
currents, where it will begin skipping pulses. The effi-
ciency in pulse skip mode can be improved slightly by
connecting 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 LTC3548 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 LTC3548 incorporates
an Under-Voltage Lockout circuit which shuts down the
part when the input voltage drops below about 1.65V.
APPLICATIO S I FOR ATIO
A general LTC3548 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 ∆I_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 ∆I_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
∆I_L = 0.3 • I_OUT(MAX), where I_OUT(MAX) is 800mA for
channel 1 and 400mA for channel 2. The largest ripple
current ∆I_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
3548f
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