ltc3405a-1.375 Linear Technology Corporation, ltc3405a-1.375 Datasheet - Page 10

• Manufacturer Part Number: ltc3405a-1.375
• Description: 1.375v, 1.5mhz, 300ma Synchronous Step-down Regulators In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
where P
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T
where T
As an example, consider the LTC3405A-1.375 with an
input voltage of 2.7V, a load current of 300mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the R
channel switch at 70°C is approximately 0.94Ω and the
R
mately 0.75Ω.
The series resistance looking into the SW pin is:
Therefore, power dissipated by the part is:
For the SOT-23 package, the θ
junction temperature of the regulator is:
which is well below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction tempera-
ture 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
discharge C
The regulator loop then acts to return V
state value. During this recovery time V
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
LTC3405A-1.375
10
DS(ON)
T
R
P
T
J
J
D
SW
= T
= 70°C + (0.0765)(250) = 89.1°C
= I
= 0.95Ω (0.51) + 0.75Ω (0.49) = 0.85Ω
D
A
of the N-channel synchronous switch is approxi-
A
LOAD
is the power dissipated by the regulator and θ
is the ambient temperature.
+ T
LOAD
OUT
R
2
• R
, which generates a feedback error signal.
OUT
• ESR), where ESR is the effective series
SW
U
. ∆I
OUT
= 76.5mW
LOAD
immediately shifts by an amount
U
J
, is given by:
also begins to charge or
JA
is 250°C/ W. Thus, the
W
OUT
OUT
DS(ON)
can be moni-
to its steady-
U
of the P-
DS(ON)
JA
).
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3405A-1.375. These items are also illustrated graphi-
cally in Figures 3 and 4. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
2. Does the (+) plate of C
3. Keep the (–) plates of C
Design Example
As a design example, assume the LTC3405A-1.375 is
used in a single lithium-ion battery-powered cellular phone
application. The V
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.15A 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
1.375V. With this information we can calculate L using
equation (1),
trace and the V
wide.
possible? This capacitor provides the AC current to the
internal power MOSFETs.
L
=
V
( )
OUT
f
Figure 3. LTC3405A-1.375 Layout Diagram
( )
–
+
1
∆
I
L
BOLD LINES INDICATE HIGH CURRENT PATHS
C
OUT
V
L1
OUT
IN
IN
trace should be kept short, direct and
will be operating from a maximum of
1
2
3
⎛
⎜
⎝
1
RUN
SW
LTC3405A-1.375
−
GND
IN
IN
V
and C
V
OUT
connect to V
IN
MODE
V
⎞
⎟
⎠
OUT
C
V
OUT
IN
IN
6
5
4
as close as possible.
IN
3405A1375 F03
as closely as
V
IN
3405a1375f
(3)