LTC3448_15 Datasheet, PDF (10/20 Pages) Linear Technology – 1.5MHz/2.25MHz, 600mA Synchronous Step-Down Regulator with LDO Mode

LTC3448
APPLICATIO S I FOR ATIO
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 gener-
ally cost more than powdered iron core inductors with
similar electrical characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3448 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3448 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
1.5
CDRH3D16
2.2
3.3
4.7
0.043
0.075
0.110
0.162
1.55
3.8 × 3.8 × 1.8
1.20
1.10
0.90
Sumida
2.2
0.116
CMD4D06
3.3
0.174
4.7
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Coilcraft
ME3220
2.2
0.104
3.3
0.138
4.7
0.190
1.8
2.5 × 3.2 × 2.0
1.3
1.2
Murata
LQH3C
1.0
0.060
2.2
0.097
4.7
0.150
1.00
2.5 × 3.2 × 2.0
0.79
0.65
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:
[ ^{( )}]^1/2
V_OUT V_IN − V_OUT
C_IN required I_RMS ≅ I_OMAX
V_IN
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. 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
10
temperature than required. Always consult the manufac-
turer 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 for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. In any
case, if LDO mode is enabled, the value of C_OUT must have
a minimum value of 2µF to ensure loop stability. The
output ripple ∆V_OUT is determined by:
∆V_OUT
≅
⎛
∆I_{L ⎝⎜}ESR
+
1
8fC _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.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3448’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V_IN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
3448f