LTC3546 Datasheet, PDF (17/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
APPLICATIONS INFORMATION
density, but it has a larger ESR and it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface tantalums, available in case heights ranging from
2mm to 4mm. Aluminum electrolytic capacitors have a
significantly larger ESR, and are often used in extremely
cost-sensitive applications provided that consideration
is given to ripple current ratings and long term reliability.
Ceramic capacitors have the lowest ESR and cost but also
have the lowest capacitance density, high voltage and
temperature coefficient and exhibit audible piezoelectric
effects. In addition, the high Q of ceramic capacitors along
with trace inductance can lead to significant ringing. Other
capacitor types include the Panasonic specialty polymer
(SP) capacitors.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Because the
LTC3546 control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size. When choosing the input and output
ceramic capacitors, choose the X5R or X7R dielectric
formulations. These dielectrics have the best temperature
and voltage characteristics of all the ceramics for a given
value and size.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the V_IN pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
disrupt circuit operation or damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation com-
ponents and the output capacitor size. Typically, 3 to 4
cycles are required to respond to a load step, but only in
the first cycle does the output drop linearly. The output
droop, V_DROOP, is usually about 2 to 3 times the linear
droop of the first cycle. Thus, a good place to start is with
the output capacitor size of approximately:
C_OUT


2.5
I_OUT
f_OV_DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since impedance to
the supply is very low. A 10μF ceramic capacitor is usually
enough for these conditions.
Setting the Output Voltage
The LTC3546 generates a 0.6V reference voltage between
the feedback pin, V_FB1 and V_FB2, and the signal ground.
The output voltage is set by a resistive divider according
to the following formula:
V_OUT1


0.6V
₁₊
R1
R2


V_OUT2


0.6V
₁₊
R3
R4


Resistor locations are shown in Figure 2.
V_OUT1
V_OUT2
C_FF1
R1
R3
LTC3546
V_FB1
R2
V_FB2
R4 3546 F02
C_FF2
Figure 2. Setting Output Voltages
3546fb
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