LTC3642
APPLICATIONS INFORMATION
ThebasicLTC3642applicationcircuitisshownonthefront
page of this data sheet. External component selection is
determinedbythemaximumloadcurrentrequirementand
beginswiththeselectionofthepeakcurrentprogramming
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
margintoprovideadequateloadcurrentunderallforesee-
able operating conditions.
resistor,R .TheinductorvalueLcanthenbedetermined,
ISET
followed by capacitors C and C
.
IN
OUT
Inductor Selection
Peak Current Resistor Selection
The inductor, input voltage, output voltage and peak cur-
rent 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:
Thepeakcurrentcomparatorhasamaximumcurrentlimit
of 115mA nominally, which results in a maximum aver-
age 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.
⎛
⎞ ⎛
⎞
VOUT
f •I
VOUT
L =
• 1–
⎟ ⎜
PEAK ⎠ ⎝
⎜
⎟
V
⎝
⎠
IN
The threshold can be easily programmed with an ap-
The variation in switching frequency with input voltage
and inductance is shown in the following two figures for
propriately chosen resistor (R ) between the I
pin
ISET
SET
and ground. The value of resistor for a particular peak
current can be computed by using Figure 1 or the follow-
ing equation:
typical values of V . For lower values of I
, multiply
OUT
PEAK
the frequency in Figure 2 and Figure 3 by 115mA/I
.
PEAK
An additional constraint on the inductor value is the
LTC3642’s100nsminimumon-timeofthehighsideswitch.
Therefore, in order to keep the current in the inductor well
controlled, the inductor value must be chosen so that it is
6
R
= I
• 9.09 • 10
PEAK
ISET
where 25mA < I
< 115mA.
PEAK
The peak current is internally limited to be within the
larger than L , which can be computed as follows:
MIN
range of 25mA to 115mA. Shorting the I pin to ground
SET
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
V
IN(MAX) • tON(MIN)
LMIN
=
IPEAK(MAX)
where V
is the maximum input supply voltage for
IN(MAX)
the application, t
1100
1000
900
800
700
600
500
400
300
200
100
0
is 100ns, and I
is the
ON(MIN)
PEAK(MAX)
maximum allowed peak inductor current. Although the
above equation provides the minimum inductor value,
higherefficiencyisgenerallyachievedwithalargerinductor
value, which produces a lower switching frequency. For a
given inductor type, however, as inductance is increased
DC resistance (DCR) also increases. Higher DCR trans-
lates 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
3642f
10
20 25 30 35 40 45 50
MAXIMUM LOAD CURRENT (mA)
15
3642 F01
Figure 1. RISET Selection
10
Linear [ Linear ]
