ltc3406ab-2 Linear Technology Corporation, ltc3406ab-2 Datasheet - Page 11

• Manufacturer Part Number: ltc3406ab-2
• Description: 2.25mhz, 600ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

The junction temperature, T
where T
As an example, consider the LTC3406AB-2 in dropout
at an input voltage of 2.7V, a load current of 600mA and
an ambient temperature of 70°C. From the typical per-
formance graph of switch resistance, the R
P-channel switch at 70°C is approximately 0.27Ω. There-
fore, power dissipated by the part is:
For the SOT-23 package, the θ
junction temperature of the regulator is:
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, 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 (ΔI
resistance of C
charge C
regulator loop then acts to return V
value. During this recovery time V
for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
APPLICATIONS INFORMATION
T
P
T
J
J
D
= T
= 70°C + (0.0972)(250) = 94.3°C
= I
A
A
LOAD
OUT
is the ambient temperature.
+ T
LOAD
, which generates a feedback error signal. The
R
2
• R
OUT
• ESR), where ESR is the effective series
DS(ON)
. ΔI
OUT
LOAD
immediately shifts by an amount
= 97.2mW
also begins to charge or dis-
J
, is given by:
JA
is 250°C/ W. Thus, the
OUT
OUT
can be monitored
to its steady-state
DS(ON)
DS(ON)
of the
).
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 paral-
lel with C
can 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
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
LTC3406AB-2. These items are also illustrated graphically
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 V
3. Does C
4. Keep the switching node, SW, away from the sensitive
5. Keep the (–) plates of C
trace, the V
short, direct and wide.
resistors? The resistive divider R1/R2 must be
connected between the (+) plate of C
capacitor provides the AC current to the internal
power MOSFETs.
V
close as possible.
FB
node.
LOAD
OUT
IN
connect to V
). Thus, a 10μF capacitor charging to 3.3V
, causing a rapid drop in V
OUT
FB
trace, and the V
pin connect directly to the feedback
IN
IN
as closely as possible? This
, C
LTC3406AB-2
OUT
IN
and the IC ground as
trace should be kept
OUT
OUT
. No regulator
and ground.
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