Datasheet

LTC3642
10
3642fc
APPLICATIONS INFORMATION
The basic LTC3642 application circuit is shown on the front
page of this data sheet. External component selection is
determined by the maximum load current requirement and
begins with the selection of the peak current programming
resistor, R
ISET
. The inductor value L can then be determined,
followed by capacitors C
IN
and C
OUT
.
Peak Current Resistor Selection
The peak current comparator has a maximum current
limit of 115mA nominally, which results in a maximum
average current of 55mA. For applications that demand
less current, the peak current threshold can be reduced
to as little as 25mA. This lower peak current allows the
use of lower value, smaller components (input capacitor,
output capacitor and inductor), resulting in lower input
supply ripple and a smaller overall DC/DC converter.
The threshold can be easily programmed with an ap-
propriately chosen resistor (R
ISET
) between the I
SET
pin
and ground. The value of resistor for a particular peak
current can be computed by using Figure 1 or the follow-
ing equation:
R
ISET
= I
PEAK
• 9.09 • 10
6
where 25mA < I
PEAK
< 115mA.
The peak current is internally limited to be within the
range of 25mA to 115mA. Shorting the I
SET
pin to ground
programs the current limit to 25mA, and leaving it floating
sets the current limit to the maximum value of 115mA.
When selecting this resistor value, be aware that the
Figure 1. R
ISET
Selection
maximum average output current for this architecture
is limited to half of the peak current. Therefore, be sure
to select a value that sets the peak current with enough
margin to provide adequate load current under all foresee-
able operating conditions.
Inductor Selection
The inductor, input voltage, output voltage and peak current
determine the switching frequency of the LTC3642. For
a given input voltage, output voltage and peak current,
the inductor value sets the switching frequency when the
output is in regulation. A good first choice for the inductor
value can be determined by the following equation:
L =
V
OUT
f • I
PEAK






• 1–
V
OUT
V
IN






The variation in switching frequency with input voltage
and inductance is shown in the following two figures for
typical values of V
OUT
. For lower values of I
PEAK
, multiply
the frequency in Figure 2 and Figure 3 by 115mA/I
PEAK
.
An additional constraint on the inductor value is the
LTC3642’s 100ns minimum on-time of the high side switch.
Therefore, in order to keep the current in the inductor well
controlled, the inductor value must be chosen so that it is
larger than L
MIN
, which can be computed as follows:
L
MIN
=
V
IN(MAX)
• t
ON(MIN)
I
PEAK(MAX)
where V
IN(MAX)
is the maximum input supply voltage for
the application, t
ON(MIN)
is 100ns, and I
PEAK(MAX)
is the
maximum allowed peak inductor current. Although the
above equation provides the minimum inductor value,
higher efficiency is generally achieved with a larger inductor
value, which produces a lower switching frequency. For a
given inductor type, however, as inductance is increased
DC resistance (DCR) also increases. Higher DCR translates
into higher copper losses and lower current rating, both
of which place an upper limit on the inductance. The
recommended range of inductor values for small surface
mount inductors as a function of peak current is shown in
Figure 4. The values in this range are a good compromise
between the tradeoffs discussed above. For applications
MAXIMUM LOAD CURRENT (mA)
10
R
ISET
(k)
300
900
1000
1100
20
30
35
3642 F01
100
700
500
200
800
0
600
400
15
25
40
50
45