LTC3408_15 Datasheet, PDF (8/12 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor

LTC3408
APPLICATIO S I FOR ATIO
At output voltages below 0.6V, the switching frequency
decreases linearly to a minimum of approximately 700kHz.
This places the maximum ripple current (in forced con-
tinuous mode) at the highest input voltage and the lowest
output voltage. In practice, the resulting ouput ripple
voltage is 10mV to 15mV using the components specified
in Figure 1.
The DC current rating of the inductor should be at least equal
to the maximum load current plus half the ripple current to
prevent core saturation. Thus, a 660mA rated inductor
should be enough for most applications (600mA + 60mA).
For better efficiency, choose a low DC-resistance inductor.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy but generally
cost more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more on the price versus size
requirements and any radiated field/EMI requirements
than on what the LTC3408 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3408 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (ΩMAX) CURRENT (A) WxLxH (mm³)
Sumida
CDRH2D11
4.7
0.135
0.5
3.2 x 3.2 x 1.2
Sumida
4.7
CDRH2D18/LD
0.078
0.63
3.2 x 3.2 x 2.0
Sumida
CMD4D06
4.7
0.216
0.75
3.5 x 4.1 x 0.8
Murata
LQH32C
4.7
0.150
0.65
2.5 x 3.2 x 2.0
Taiyo Yuden
4.7
0.250
0.210
1.6 x 2.0 x 1.6
LBLQ2016
Toko
D312C
4.7
0.20
0.79
3.6 x 3.6 x 1.2
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. To prevent large
voltage transients, a low ESR input capacitor sized for the
8
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
C_IN
required
I_RMS
≅
I_OMAX
[V_OUT (V_IN – V_OUT )]^1/2
V_IN
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
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 re-
quired. 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
requirement for C_OUT has been met, the RMS current
rating generally far exceeds the I_RIPPLE(P-P) requirement.
The output ripple V_OUT is determined by:
∆V_OUT
≅

∆I_{L}ESR +
8f
1
C _OUT


where f = operating frequency, C_OUT = output capacitance
and I_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since I_L increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, 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 tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
The bulk capacitance values in Figure 1(a) (C_IN = 10µF,
C_OUT = 4.7µF) are tailored to mobile phone applications, in
which the output voltage is expected to slew quickly
according to the needs of the power amplifier. Holding the
output capacitor to 4.7µF facilitates rapid charging and
discharging. When the output voltage descends quickly in
3408f