LTC3408 LINER [Linear Technology], LTC3408 Datasheet - Page 11

• Manufacturer Part Number: LTC3408
• Description: 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
dissipate (0.531A)
(0.0143 + 0.0425)(43) = 71.1°C.
Reductions in power dissipation occur at higher supply
voltages, where the junction temperature is lower due to
reduced switch resistance (R
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to (I
resistance of C
charge C
regulator loop then acts to return V
value. During this recovery time V
overshoot or ringing that would indicate a stability prob-
lem. For a detailed explanation of switching control loop
theory, see Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 • C
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
V
IN
OUT
OUT
BOLD LINES INDICATE HIGH CURRENT PATHS
, causing a rapid drop in V
C
LOAD
IN
, which generates a feedback error signal. The
OUT
• ESR), where ESR is the effective series
Figure 5. Layout Diagram
1
2
3
4
2
. I
U
• 0.08Ω = 22.6mW. Thus, T
V
GND
OUT
V
SW
LOAD
IN
OUT
LTC3408
immediately shifts by an amount
U
also begins to charge or dis-
V
RUN
REF
OUT
V
IN
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
DS(ON)
8
7
6
5
OUT
W
OUT
OUT
).
can be monitored for
. No regulator can
to its steady state
3403 F05
C
C
R
OUT
REF
REF
U
J
= 70°C +
DAC
V
LOAD
OUT
).
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3408. These items are also illustrated graphically in
Figures 5 and 6. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V
2. Does the (+) plate of C
possible? This capacitor provides the AC drive to the
internal power MOSFETs.
3. Keep the (–) plates of C
Design Example
As a design example, assume the LTC3408 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The V
down to about 2.7V. The load current requirement is a
maximum of 0.6A but most of the time it will be in standby
mode, requiring only 2mA. Efficiency at both low and high
load currents is important. Output voltage is 2.5V. With
this information we can calculate L using Equation (1),
L
TO DAC
=
VIA TO PIN 7
( )(
f
1
∆
IN
C
I
IN
L
will be operating from a maximum of 4.2V
IN
)
R
V
REF
trace should be kept short, direct and wide.
OUT
Figure 6. Suggested Layout
VIA TO PIN 8
V
GND
OUT
SW
V



IN
1
–
1
2
3
4
IN
V
IN
V
OUT
LTC3408
and C
IN
VIA TO REF
connect to V
L1



C
OUT
OUT
8
7
6
5
as close as possible.
VIA TO V
V
V
REF
RUN
OUT
IN
VIA TO PIN 1
IN
LTC3408
IN
C
as closely as
REF
VIA TO PIN 2
VIA TO GND
11
3408 F06
3408f
(2)