ltc3445euf-pbf Linear Technology Corporation, ltc3445euf-pbf Datasheet - Page 21

• Manufacturer Part Number: ltc3445euf-pbf
• Description: I2c Controllable Buck Regulator With Two Ldos In A 4mm ? 4mm Qfn
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
As an example, consider the LTC3445 in dropout at an
input voltage of 2.7V, an ambient temperature of 70°C, a
buck load current of 600mA, LDO1 set to 1.3V with a load
of 25mA, LDO2 set to 1.1V with a load of 15mA, and the
PowerPath regulator at 2.5V with a load of 6µA. From the
typical performance graph of switch resistance, the R
of the P-channel switch at 70°C is approximately 0.52Ω.
Therefore, power dissipated by the part is:
For the QFN24 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
temperature is lower due to reduced switch resistance
(R
DS(ON)
P
P
P
P
P
T
J
D(BUCK)
D(LDO1)
D(LDO2)
D(PowerPath)
D(TOTAL)
= 70°C + (0.239)(37) = 78.8°C
).
V
V
10
CC1
CC1
= (2.7 – 1.3)V • 0.025A = 35mW
= (2.7 – 1.1)V • 0.015A = 24mW
= I
GND
C
25
= 0.239W
IN
LOAD
GND
11
= (2.7 – 2.5)V • 6µA = 1.2µW
U
2
GND
• R
NC
12
DS(ON)
Figure 9
U
RUN
SW
NC
NC
FB
17
16
15
14
13
= 180mW
JA
C
OUT
W
is 37°C/W. Thus, the
L1
OUT
VIA TO
FB
U
FB
RUN
3445 F09
SW
VIA TO
OUT
DS(ON)
PC BOARD LAYOUT CHECKLIST
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3445. These items are also illustrated graphically in
Figures 9 and 10. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
2. Does the FB pin connect directly to the output voltage
3. Does the (+) plate of C
4. Keep the switching node, SW, away from the sensitive
5. Keep the (–) plates of C
trace, the V
short, direct and wide.
reference? Ensure that there is no load current running
from the reference voltage and the FB pin.
possible? This capacitor provides the AC current to the
internal power MOSFETs.
FB node.
V
V
10
CC1
CC1
CC1
GND
C
25
IN
trace and the V
BOLD LINES INDICATE HIGH CURRENT PATH
GND
11
Figure 10
IN1
C
IN
NC
OUT
12
and C
connect to V
RUN
SW
NC
NC
FB
17
16
15
14
13
CC2
OUT
trace should be kept
as close as possible.
CC1
LTC3445
L1
3445 f10
as closely as
V
OUT
21
3445fa