ltc3405a Linear Technology Corporation, ltc3405a Datasheet - Page 11

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

APPLICATIO S I FOR ATIO
The junction temperature, T
where T
As an example, consider the LTC3405A in dropout at 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
P-channel switch at 70°C is approximately 0.94Ω. There-
fore, 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
T
P
T
J
J
D
= T
= 70°C + (0.0846)(250) = 91.15°C
= I
A
A
LOAD
is the ambient temperature.
+ T
2
R
• R
DS(ON)
U
OUT
immediately shifts by an amount
= 84.6mW
U
J
, is given by:
JA
is 250°C/ W. Thus, the
W
V
OUT
BOLD LINES INDICATE HIGH CURRENT PATHS
–
+
Figure 5. LTC3405A Layout Diagram
DS(ON)
C
OUT
U
L1
DS(ON)
of the
1
2
3
RUN
SW
).
GND
LTC3405A
MODE
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.
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.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3405A. These items are also illustrated graphically in
Figures 5 and 6. Check the following in your layout:
V
C
FB
IN
IN
6
5
4
OUT
, causing a rapid drop in V
R2
LOAD
OUT
, which generates a feedback error signal.
OUT
C
• ESR), where ESR is the effective series
3405A F05
FWD
. ∆I
R1
LOAD
V
IN
also begins to charge or
OUT
LTC3405A
. No regulator can
OUT
OUT
can be moni-
to its steady-
11
LOAD
3405afa
).