LTC3603 Linear Dimensions Semiconductor, LTC3603 Datasheet - Page 10

• Manufacturer Part Number: LTC3603
• Description: Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Dimensions Semiconductor

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APPLICATIONS INFORMATION
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated fi eld/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
C
The input capacitance, C
dal current at the source of the top MOSFET. To prevent
large ripple voltage, a low ESR input capacitor sized for
the maximum RMS current should be used. RMS current
is given by:
This formula has a maximum at V
I
monly used for design because even signifi cant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life which 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.
The selection of C
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, ΔV
The output ripple is highest at maximum input voltage
since ΔI
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, special
polymer, aluminum electrolytic and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
LTC3603
10
RMS
IN
I
Δ
RMS
and C
V
= I
OUT
L
=
OUT
increases with input voltage. Multiple capacitors
I
OUT
OUT MAX
≤ Δ
/2. This simple worst-case condition is com-
I
(
Selection
L
•
⎛
⎝ ⎜
OUT
)
ESR
•
is determined by the effective series
V
OUT
V
OUT
+
IN
IN
, is determined by:
8
, is needed to fi lter the trapezoi-
fC
•
1
OUT
V
V
OUT
IN
⎞
⎠ ⎟
– 1
IN
= 2V
OUT
, where
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
signifi cantly higher ESR but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics but can have a high
voltage coeffi cient and audible piezoelectric effects. The
high Q of ceramic capacitors with trace inductance can
also lead to signifi cant ringing.
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. However, care must
be taken when these capacitors are used at the input and
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
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
The resistive divider allows the V
of the output voltage as shown in Figure 1.
IN
V
. At best, this ringing can couple to the output and be
OUT
=
0 6
.
Figure 1. Setting the Output Voltage
V
•
⎛
⎝ ⎜
IN
1
+
large enough to damage the part.
R
R
LTC3603
2
1
⎞
⎠ ⎟
SGND
3603 F01
V
FB
FB
V
pin to sense a fraction
OUT
R2
R1
3603f