LTC3412 LINER [Linear Technology], LTC3412 Datasheet - Page 13

• Manufacturer Part Number: LTC3412
• Description: 2.5A, 4MHz, Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The junction temperature, T
where T
As an example, consider the LTC3412 in dropout at an
input voltage of 3.3V, a load current of 2.5A 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 97mΩ. There-
fore, power dissipated by the part is:
For the TSSOP 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 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 ΔI
resistance of C
discharge C
the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability prob-
lem. The I
tor shown in Figure 1 will provide adequate compensation
for most applications.
Design Example
As a design example, consider using the LTC3412 in an
application with the following specifications: V
4.2V, V
= 1MHz. Because efficiency is important at both high and
low load current, Burst Mode operation will be utilized.
T
P
T
J
J
D
= T
= 70°C + (0.61W)(37.6°C/W) = 93°C
= (I
OUT
A
A
LOAD
is the ambient temperature.
TH
+ T
LOAD
= 2.5V, I
OUT
pin external components and output capaci-
R
2
)(R
generating a feedback error signal used by
(ESR), where ESR is the effective series
OUT
DS(ON)
U
. ΔI
OUT(MAX)
OUT
LOAD
immediately shifts by an amount
) = (2.5A)
U
OUT
J
, is given by:
= 2.5A, I
also begins to charge or
OUT
to its steady-state value.
JA
is 37.6°C/W. Thus the
can be monitored for
2
W
(97mΩ) = 0.61W
OUT(MIN)
DS(ON)
IN
= 10mA, f
U
of the P-
= 2.7V to
DS(ON)
).
First, calculate the timing resistor:
Use a standard value of 309k. Next, calculate the inductor
value for about 40% ripple current at maximum V
Using a 1μH inductor, results in a maximum ripple current
of:
C
satisfy the output voltage ripple requirement and the bulk
capacitance needed for loop stability. In this application,
two tantalum capacitors will be used to provide the bulk
capacitance and a ceramic capacitor in parallel to lower the
total effective ESR. For this design, two 100μF tantalum
capacitors in parallel with a 10μF ceramic capacitor will be
used. C
Decoupling the PV
capacitor and a 220μF tantalum capacitor is adequate for
most applications.
The burst clamp and output voltage can now be pro-
grammed by choosing the values of R1, R2 and R3. The
voltage on the MODE pin will be set to 0.32V by the resistor
divider consisting of R2 and R3. A burst clamp voltage of
0.32V will set the minimum inductor current, I
follows:
OUT
R
I
I
L
Δ =
RMS
BURST
OSC
I
=
L
will be selected based on the ESR that is required to
⎛
⎜
⎝
IN
=
(
=
1
⎛
⎜
⎝
should be sized for a maximum current rating of:
MHz A
(
=
(
2 5
3 23 10
2 5
1
(
MHz
.
.
.
0 32
1 10
A
.
V
)( )
2 5
•
)
.
1
•
⎛
⎜
⎝
)(
V
V
2 5
4 2
6
1
IN
⎞
⎟
⎠
μ
−
.
.
11
H
and SV
⎛
⎜
⎝
V
V
0 2
1
)
−
.
⎞
⎟
⎠
⎞
⎟
⎠
−
V
10
⎛
⎜
⎝
2 5
4 2
1
)
4 2
2 5
k
.
.
⎛
⎜
⎝
−
.
.
IN
3 75
=
V
V
0 8
2 5
4 2
V
V
.
pins with a 22μF ceramic
.
⎞
⎟ =
⎠
313
.
.
− =
V
V
V
V
1 1 23
1 01
⎞
⎟ =
⎠
⎞
⎟ =
⎠
k
.
1 01
563
LTC3412
μ
.
.
H
A
mA
A
RMS
BURST
13
IN
:
3412fb
, as