Datasheet

LTC3542
12
3542fa
parallel with C
OUT
, causing a rapid drop in V
OUT
. No regula-
tor can deliver enough current to prevent this problem, if
the switch connecting the load has low resistance and is
driven quickly. The solution is to limit the turn-on speed of
the load switch driver. A Hot Swap
TM
controller is designed
specifi cally for this purpose and usually incorporates cur-
rent limit, short circuit protection and soft-start.
Design Example
As a design example, assume the LTC3542 is used in a single
lithium-ion battery-powered cellular phone application. The
V
IN
will be operating from a maximum of 4.2V down to
about 2.7V. The load current requirement is a maximum
of 0.5A, but most of the time it will be in standby mode,
requiring only 2mA. Effi ciency at both low and high load
currents is important. Output voltage is 1.8V.
With this information we can calculate L using:
L
fI
V
V
V
L
OUT
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
1
1
•
••–
Δ
Substituting V
OUT
= 1.8V, V
IN
= 4.2V, ΔI
L
= 200mA and
f = 2.25MHz gives:
L
V
MHz mA
V
V
μH=
⎛
⎝
⎜
⎞
⎠
⎟
=
18
2 25 200
1
18
42
228
.
.•
•–
.
.
.
Choosing a vendor’s closest inductor value of 2.2μH results
in a maximum ripple current of:
ΔI
V
MHz μH
V
V
L
=
⎛
⎝
⎜
⎞
⎠
⎟
=
18
225 22
1
18
42
207
.
.•.
•–
.
.
.88mA
Hot Swap is a trademark of Linear Technology Corporation.
C
IN
will require an RMS current rating of at least
0.25A ≅ I
LOAD(MAX)
/2 at temperature and C
OUT
will require
ESR of less than 0.2Ω. In most cases, ceramic capacitors
will satisfy these requirements. Select C
OUT
= 10μF and
C
IN
= 10μF.
For the feedback resistors, choose R1 = 75k, R2 can be
calculated from:
R
V
V
R
V
V
k
OUT
2
06
11
18
06
175=
⎛
⎝
⎜
⎞
⎠
⎟
=
⎛
⎝
⎜
⎞
⎠
⎟
.
–•
.
.
–• ==150k
Figure 3 shows the complete circuit along with its effi ciency
curve, load step response and recommended layout
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3542. These items are also illustrated graphically
in Figure 3b. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V
IN
trace should be kept short, direct and
wide.
2. Does the V
FB
pin connect directly to the feedback re-
sistors? The resistive divider R1/R2 must be connected
between the (+) plate of C
OUT
and ground.
3. Does the (+) plate of C
IN
connect to V
IN
as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the (–) plates of C
IN
and C
OUT
as close as pos-
sible.
5. Keep the switching node, SW, away from the sensitive
V
FB
node.
APPLICATIONS INFORMATION