LTC3546 Datasheet, PDF (15/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
OPERATION
clamp level. When the BMC pin is pulled to V_IN, an internal
Burst Mode clamp level is used.
For lower output voltage ripple at low currents, pulse
skipping mode can be used. In this mode, the LTC3546
continues to switch at constant frequency down to very low
currents, where it will eventually begin skipping pulses.
Finally, in forced continuous mode, the inductor current
is constantly cycled which creates a fixed output voltage
ripple at all output current levels. This feature is desirable in
telecommunications since the noise is a constant frequency
and is thus easy to filter out. Another advantage of this mode
is that the regulator is capable of both sourcing current into
a load and sinking some current from the output. In forced
continuous operation, an overvoltage comparator monitors
the V_FB pin and decreases the currrent limit whenever an
overvoltage condition is detected (V_FB > 0.63V).
The SYNC/MODE pin selects what mode the LTC3546 is in.
The SYNC/MODE pin sets the mode for both regulators.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases to 100% which is the
dropout condition. In the dropout condition, 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 MOSFETs and inductors.
Low Supply Operation
The LTC3546 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.14V to prevent unstable operation.
APPLICATIONS INFORMATION
A general LTC3546 application circuit is shown in Figure 7.
External component selection is driven by the load require-
ment, and begins with the selection of the inductors L1,
and L2. Once L1 and L2 are chosen, C_IN, C_OUT1, and C_OUT2
can be selected.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f_O, of the LTC3546 is determined
by pulling the FREQ pin to V_IN, for 2.25MHz operation, by
connecting an external resistor from FREQ to ground, or
by driving an external clock signal into SYNC/MODE.
When using an external resistor to set the oscillator fre-
quency use the following equation:
R_T
=
2.51• 10¹¹
f_O
(Ω)
–
20kΩ
for 0.75MHz ≤ f_O ≤ 4MHz. Or use Figure 1 to select the
value for R_T.
The maximum operating frequency is also constrained
by the minimum on-time (typically 70ns) and duty cycle,
especially when forced continuous mode is selected.
Assuming a worst case minimum on-time of 150ns, this
can be calculated as:
( ) f_O(MAX)
≈
⎛
6.67 _⎜
⎝
V_OUT
V_IN(MAX)
⎞
⎟
⎠
MHz
500
450
400
350
300
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
FREQUENCY (MHz)
3546 F01
Figure 1. Frequency vs R_T
3546f
15