Datasheet

LTC3101
21
3101fb
APPLICATIONS INFORMATION
The basic LTC3101 application circuit is shown as the
Typical Application on the front page of this data sheet.
The external component selection is dependent upon the
required performance of the IC in each particular appli-
cation given considerations and tradeoffs such as PCB
area, output voltages, output currents, ripple voltages
and effi ciency. This section of the data sheet provides
some basic guidelines and considerations to aid in the
selection of external components and the design of the
application circuit.
C
RS
Capacitor Selection
A capacitor from the C
RS
pin to ground is used to pro-
gram the duration of the microprocessor reset signal on
the RESET pin. A low leakage ceramic capacitor should
be utilized to ensure reliable temperature independent
operation. At the start of the active-low reset pulse, a 1A
(typical) current begins charging the C
RS
capacitor. The
RESET pulse ends when the voltage at the C
RS
pin reaches
1.20V (typical). Therefore, the required C
RS
capacitor value,
C
RS
, is given by the following equation where t
RESET
is the
desired reset duration in milliseconds:
C
t
µF
RS
RESET
=
()
1200

If the microprocessor reset function of the LTC3101 is
unused, the C
RS
pin can be left unconnected.
LDO Output Capacitance
The LDO has been specifi cally designed for stable opera-
tion with a wide range of output capacitors. For most ap-
plications, a low ESR ceramic capacitor of at least 4.7µF
should be utilized. Large valued supercapacitors can be
connected directly to the LDO output without requiring a
series isolation resistor for loop stability. However, if the
supercapacitor has signifi cant ESR, it may be necessary
to place a small 4.7F ceramic in parallel with the super-
capacitor to maintain an adequate phase margin.
MAX Capacitor Selection
The MAX output serves as the input to the LDO. There-
fore, even if the MAX output is unused directly in the
application, it is recommended that it be bypassed with
a 1F or larger ceramic capacitor. There is no limit to the
maximum capacitance on this pin. However, the soft-start
duration is formed by the current-limited output charg-
ing the capacitance attached to the pin so larger output
capacitors will result in proportionally longer soft-start
durations.
Buck Inductor Selection
The choice of buck inductor value infl uences both the ef-
fi ciency and the magnitude of the output voltage ripple.
Larger inductance values will reduce inductor current
ripple and will therefore lead to lower output voltage
ripple. For a fi xed DC resistance, a larger value inductor
will yield higher effi ciency by lowering the peak current to
be closer to the average output current. However, a larger
inductor within a given inductor family will generally have
a greater series resistance, thereby counteracting this
effi ciency advantage.
Given a desired peak-to-peak current ripple, ΔI
L
, the
required inductance can be calculated via the following
expression, where f represents the switching frequency
in MHz:
L =
V
OUT
f•I
L
1–
V
OUT
V
IN






μH
()
A reasonable choice for ripple current is ΔI
L
= 140mA
which represents 40% of the maximum 350mA load
current. The DC current rating of the inductor should be
at least equal to the maximum load current plus half the
ripple current in order to prevent core saturation and loss
of effi ciency during operation. To optimize effi ciency the
inductor should have a low DC resistance (DCR).
In particularly space-restricted applications it may be
advantageous to use a much smaller value inductor at
the expense of larger ripple current. In such cases, the
converter will operate in discontinuous conduction for a
wider range of output loads and effi ciency will be reduced.
In addition, there is a minimum inductor value required
to maintain stability of the current loop as determined by
the fi xed internal slope compensation. Specifi cally, if the