LTC3445 datasheet(21/24 Pages) LINER

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LTC3445

3445fa

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 6

µA. From the

typical performance graph of switch resistance, the RDS(ON)

of the P-channel switch at 70

°C is approximately 0.52Ω.

Therefore, power dissipated by the part is:

PD(BUCK) = ILOAD 2 • RDS(ON) = 180mW

PD(LDO1) = (2.7 – 1.3)V • 0.025A = 35mW

PD(LDO2) = (2.7 – 1.1)V • 0.015A = 24mW

PD(PowerPath) = (2.7 – 2.5)V • 6µA = 1.2µW

PD(TOTAL) = 0.239W

For the QFN24 package, the

θJA is 37°C/W. Thus, the

junction temperature of the regulator is:

TJ = 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

(RDS(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 VCC1 trace and the VCC2 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 CIN1 connect to VCC1 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 CIN and COUT as close as possible.

APPLICATIO S I FOR ATIO

Figure 9

Figure 10

VCC1

GND

OUT

3445 F09

RUN

GND

RUN

VIA TO

OUT

VIA TO

CIN

COUT

VCC1

3445 f10

GND

VOUT

BOLD LINES INDICATE HIGH CURRENT PATH

VCC1

CIN

GND

RUN

COUT

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