LTC3445_15 Datasheet, PDF (21/24 Pages) Linear Technology – I2C Controllable Buck Regulator with Two LDOs

LTC3445
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_DS(ON)
of the P-channel switch at 70°C is approximately 0.52Ω.
Therefore, power dissipated by the part is:
P_D(BUCK) = I_LOAD ² • R_DS(ON) = 180mW
P_D(LDO1) = (2.7 – 1.3)V • 0.025A = 35mW
P_D(LDO2) = (2.7 – 1.1)V • 0.015A = 24mW
P_D(PowerPath) = (2.7 – 2.5)V • 6µA = 1.2µW
P_D(TOTAL) = 0.239W
For the QFN24 package, the θ_JA is 37°C/W. Thus, the
junction temperature of the regulator is:
T_J = 70°C + (0.239)(37) = 78.8°C
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)).
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
trace, the V_CC1 trace and the V_CC2 trace should be kept
short, direct and wide.
2. Does the FB pin connect directly to the output voltage
reference? Ensure that there is no load current running
from the reference voltage and the FB pin.
3. Does the (+) plate of C_IN1 connect to V_CC1 as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
FB node.
5. Keep the (–) plates of C_IN and C_OUT as close as possible.
GND
25
V_CC1 GND
NC
10
11
12
C_IN
V_CC1
GND
FB
17
NC
16
RUN
15
SW
14
NC
13
C_OUT
Figure 9
FB
VIA TO
OUT
RUN
SW
L1
VIA TO
FB
OUT
3445 F09
FB
17
NC
16
GND
RUN
25
15
SW
14
NC
13
V_CC1 GND
NC
L1
10
11
12
C_IN
C_OUT
3445 f10 ^VOUT
V_CC1 BOLD LINES INDICATE HIGH CURRENT PATH
Figure 10
3445fa
21