LTC3407EMSE-2 Linear Technology, LTC3407EMSE-2 Datasheet - Page 9

• Manufacturer Part Number: LTC3407EMSE-2
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,TSSOP,10PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Output Capacitor (C
The selection of C
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering. The output ripple (∆V
mined by:
where f = operating frequency, C
and ∆I
is highest at maximum input voltage since ∆I
with input voltage. With ∆I
will be less than 100mV at maximum V
with:
Once the ESR requirements for C
RMS current rating generally far exceeds the I
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi-
num electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages. The
OS-CON semiconductor dielectric capacitor available from
Sanyo has the lowest ESR(size) product of any aluminum
electrolytic at a somewhat higher price. Special polymer
capacitors, such as Sanyo POSCAP, Panasonic Special
Polymer (SP), and Kemet A700, offer very low ESR, but
have a lower capacitance density than other types. Tanta-
lum 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 signifi-
cantly 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 tempera-
ture coefficient, and exhibit audible piezoelectric effects.
ESR
∆
V
OUT
L
COUT
= ripple current in the inductor. The output ripple
≈ ∆
< 150mΩ
I ESR
L
⎛
⎜
⎝
OUT
U
OUT
+
is driven by the required ESR to
) Selection
8
f C
U
O OUT
L
= 0.3 • I
1
OUT
OUT
⎞
⎟
⎠
W
= output capacitance
LIM
IN
have been met, the
and f
the output ripple
OUT
O
L
= 2.25MHz
RIPPLE(P-P)
) is deter-
U
increases
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 LTC3407-3 in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing 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 instrumen-
tal in giving acceptable loop phase margin. Ceramic ca-
pacitors 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
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.
V
OUT2
V
IN
Figure 2. LTC3407-3 General Schematic
C
OUT2
L2 µH
10µF
MODE/SYNC
SW2
V
RUN2
OUT2
LTC3407-3
IN
GND
V
IN
pin. At best, this ringing can
RUN1
V
SW1
POR
OUT1
LTC3407-3
L1 µH
100k
RESET
34073 F02
C
V
OUT1
OUT1
34073fa
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