LTC3602_15 Datasheet, PDF (10/20 Pages) Linear Technology – 2.5A, 10V, Monolithic Synchronous Step-Down Regulator

LTC3602
APPLICATIONS INFORMATION
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoi-
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:
I_RMS
= I_OUT(MAX)
•
V_OUT
V_IN
•
V_{IN – 1}
V_OUT
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 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_OUT is determined by the effective series
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_OUT, is determined by:
ΔV_OUT
≤
ΔI_L
•
⎛
⎝⎜
ESR
+
1
8fC_OUT
⎞
⎠⎟
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage. Multiple capacitors
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
10
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
significantly 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 coefficient and audible piezoelectric effects. The
high Q of ceramic capacitors with trace inductance can
also lead to significant 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_IN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V_IN large enough to damage the part.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
V_OUT
=
0.6V
•
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 1.
V_OUT
R2
V_FB
LTC3602
R1
SGND
3602 F01
Figure 1. Setting the Output Voltage
3602fb