LTC3404IMS8 Datasheet, PDF (10/16 Pages) Linear Technology – 1.4MHz High Efficiency Monolithic Synchronous Step-Down Regulator

LTC3404
APPLICATIO S I FOR ATIO
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Kool Mμ (from Magnetics, Inc.) is a very good, low loss
core material for toroids with a “soft” saturation character-
istic. Molypermalloy is slightly more efficient at high
(>200kHz) switching frequencies but quite a bit more
expensive. Toroids are very space efficient, especially
when you can use several layers of wire, while inductors
wound on bobbins are generally easier to surface mount.
New designs for surface mount inductors are available
from Coiltronics, Coilcraft, Dale and Sumida.
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOS-
FET is a square wave of duty cycle V_OUT/V_IN. To prevent
large voltage transients, a low ESR input capacitor sized
for the maximum RMS current must be used. The maxi-
mum RMS capacitor current is given by:
[ ^{( )}] _{CIN required IRMS ≅ IOMAX}
V_OUT V_IN − V_OUT
V_IN
1/ 2
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet size or height requirements in the
design. Always consult the manufacturer if there is any
question.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple ΔV_OUT is determined by:
ΔV_OUT
≅
⎛
ΔI_{L ⎝⎜}ESR
+
1
8fC_OUT
⎞
⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. The output ripple
10
is highest at maximum input voltage since ΔI_L increases
with input voltage. For the LTC3404, the general rule for
proper operation is:
C_OUT required ESR < 0.25Ω
The choice of using a smaller output capacitance
increases the output ripple voltage due to the frequency
dependent term but can be compensated for by using
capacitor(s) of very low ESR to maintain low ripple
voltage. The I_TH pin compensation components can be
optimized to provide stable high performance transient
response regardless of the output capacitor selected.
ESR is a direct function of the volume of the capacitor.
Manufacturers such as Taiyo-Yuden, AVX, Kemet, Sprague
and Sanyo should be considered for high performance
capacitors. The POSCAP solid electrolytic chip capacitor
available from Sanyo is an excellent choice for output bulk
capacitors due to its low ESR/size ratio. Once the ESR
requirement for C_OUT has been met, the RMS current
rating generally far exceeds the I_RIPPLE(P-P) requirement.
When using tantalum capacitors, it is critical that they are
surge tested for use in switching power supplies. A good
choice is the AVX TPS series of surface mount tantalum,
available in case heights ranging from 2mm to 4mm. Other
capacitor types include KEMET T510 and T495 series and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V_OUT = 0.8V^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
0.8V ≤ V_OUT ≤ 6V
R2
V_FB
LTC3404
R1
GND
3404 F03
Figure 3. Setting the LTC3404 Output Voltage
3404fb