LTC3407-2_15 Datasheet, PDF (9/16 Pages) Linear Technology – Dual Synchronous, 800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3407-2
APPLICATIONS INFORMATION
fer very low ESR, but have a lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density, but they have 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 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 ratings 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.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3407-2 in parallel with the
main capacitors for high frequency decoupling.
V_IN = 2.5V
TO 5.5V
V_OUT2
C_IN
BM*
PS*
RUN2 V_IN
MODE/SYNC
RUN1
POR
L2
C5
LTC3407-2
SW2
SW1
R5
POWER-ON
RESET
L1
C4
V_OUT1
R4
C_OUT2
R3
V_FB2
GND
V_FB1
R2
R1
*MODE/SYNC = 0V: PULSE-SKIPPING
MODE/SYNC = V_IN: Burst Mode OPERATION
Figure 2. LTC3407-2 General Schematic
C_OUT1
3407 F02
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 caps are
prone to temperature effects which requires 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, AVX, Kemet, 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, three to four 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 two to three 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 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3407-2 develops a 0.6V reference voltage between
the feedback pin, V_FB, and the ground as shown in Figure 2.
The output voltage is set by a resistive divider according
to the following formula:
V_OUT
=
0.6V

₁₊
R2 ^
R1
34072fc
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