LTC3548A Datasheet, PDF (11/20 Pages) Linear Technology – Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548A
APPLICATIONS INFORMATION
AVX TPS series of surface mount 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 rat-
ings and long term reliability. Ceramic capacitors have the
lowest ESR and cost, but also have the lowest capacitance
density, a 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 Special Polymer (SP) capacitors.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3548A in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. These are tempting
for switching regulator use because of their very low ESR.
Unfortunately, the ESR is so low that it can cause loop
stability problems. Solid tantalum capacitor ESR generates
a loop zero at 5kHz to 50kHz that is instrumental in giving
acceptable loop-phase margin. Ceramic capacitors remain
capacitive to beyond 300kHz and usually resonate with
their ESL before ESR becomes effective. Also, ceramic
capacitors are prone to temperature effects which require
the designer to check loop stability over the operating
temperature range. To minimize their large temperature and
voltage coefficients, only X5R or X7R ceramic capacitors
should be used. A good selection of ceramic capacitors
is available from Taiyo Yuden, TDK, and Murata.
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
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 and
the output capacitor size. Typically, 3-4 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 3 times the linear drop of the first cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
C_OUT
≈
3
f_O
ΔI_OUT
• V_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 the impedance
to the supply is very low. A 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3548A develops a 0.6V reference voltage between
the feedback pin, V_FB, and ground as shown in Figure 1.
The output voltage is set by a resistive divider according
to the following formula:
V_OUT
=
0.6V
⎛
⎝⎜
1+
R2 ⎞
R1^⎠⎟
Keeping the current small (<5μA) in these resistors maxi-
mizes efficiency, but making them too small may allow
stray capacitance to cause noise problems and reduce the
phase margin of the error amp loop.
To improve the frequency response, a feedforward capaci-
tor, C_F, may also be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
inductor or the SW line.
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