LTC3568_15 Datasheet, PDF (10/18 Pages) Linear Technology – 1.8A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3568
Applications Information
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 specialty polymer
(SP) capacitors.
In most cases, 0.1µF to 1µF of ceramic capacitors should
also be placed close to the LTC3568 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 ususally resonate with
their ESL before ESR becomes effective. Also, ceramic
caps are prone to temperature effects which requires the
designer to check loop stability over the operating tem-
perature 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 com-
ponents and the output capacitor size. Typically, 3 to 4
10
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
drop of the first cycle. Thus, a good place to start is with
the output capacitor size of approximately:
C _OUT
≈
2.5
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 22µF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3568 develops a 0.8V reference voltage between
the feedback pin, V_FB, and the signal ground as shown in
Figure 5. The output voltage is set by a resistive divider
according to the following formula:
V_OUT ≈ 0.8V ^{}1+ ^{RR21}
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 feed-forward 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.
Shutdown and Soft-Start
The SHDN/R_T pin is a dual purpose pin that sets the oscil-
lator frequency and provides a means to shut down the
LTC3568. This pin can be interfaced with control logic in
several ways, as shown in Figure 3(a) and Figure 3(b).
The I_TH pin is primarily for loop compensation, but it can
also be used to increase the soft-start time. Soft start
reduces surge currents from V_IN by gradually increasing
the peak inductor current. Power supply sequencing can
also be accomplished using this pin. The LTC3568 has an
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