LTC3407AIDD Datasheet, PDF (9/16 Pages) Linear Integrated Systems – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407
APPLICATIONS INFORMATION
capacitors, such as Sanyo POSCAP, offer very low ESR,
but have a lower capacitance density than other types.
Tantalum capacitors have the highest capacitance 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
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. 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 LTC3407 in parallel with the
main capacitors for high frequency decoupling.
V_IN = 2.5V
TO 5.5V
V_OUT2
C_IN
BURST*
RUN2 V_IN RUN1
PULSESKIP* ^MODE/SYNC
POR
L2
C5
LTC3407
SW2
SW1
R5
POWER-ON
RESET
L1
C4
V_OUT1
R4
C_OUT2
R3
V_FB2
V_FB1
GND
R2
R1
*MODE/SYNC = 0V: PULSE-SKIPPING
MODE/SYNC = V_IN: Burst Mode OPERATION
Figure 2. LTC3407 General Schematic
C_OUT1
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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 gener-
ates 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 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 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 LTC3407 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:
3407fa
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