LTC1474CS8 datasheet(12/20 Pages) LINER

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LTC1474/LTC1475

APPLICATIONS INFORMATION

Figure 8. Preventing Absolute Maximum

Ratings from Being Exceeded

1474/75 F08

VIN

VOUT

LATCHUP

PROTECTION

SCHOTTKY

LTC1474

LTC1475

with a built-in internal disconnect. The user needs to

assess his/her application to determine whether this situ-

ation could occur. If so, additional protection is necessary.

Prevention against latchup can be accomplished by sim-

ply connecting a Schottky diode across the SW and VIN

pins as shown in Figure 8. The diode will normally be

reverse biased unless VIN is pulled below VOUT at which

time the diode will clamp the (VOUT – VIN) potential to less

than the 0.6V required for latchup. Note that a low leakage

Schottky should be used to minimize the effect on no-load

supply current. Schottky diodes such as MBR0530, BAS85

and BAT84 work well. Another more serious effect of the

protection diode leakage is that at no load with nothing to

provide a sink for this leakage current, the output voltage

can potentially float above the maximum allowable toler-

ance. To prevent this from occuring, a resistor must be

connected between VOUT and ground with a value low

enough to sink the maximum possible leakage current.

Thermal Considerations

In the majority of the applications, the LTC1474/LTC1475

do not dissipate much heat due to their high efficiency.

However, in applications where the switching regulator is

running at high ambient temperature with low supply

voltage and high duty cycles, such as dropout with the

switch on continuously, the user will need to do some

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated by the regulator

exceeds the maximum junction temperature of the part.

The temperature rise is given by:

TR = P • θJA

where P is the power dissipated by the regulator and

θJA

is the thermal resistance from the junction of the die to the

ambient temperature.

The junction temperature is given by:

TJ = TA + TR

As an example consider the LTC1474/LTC1475 in dropout

at an input voltage of 3.5V, a load current of 300mA, and

an ambient temperature of 70

°C. From the typical perfor-

mance graph of switch resistance, the on-resistance of the

P-channel switch at 70

°C is 3.5Ω. Therefore, power dissi-

pated by the part is:

P = I2 • RDS(ON) = 0.315W

For the MSOP package, the

θJA is 150°C/W. Thus the

junction temperature of the regulator is:

TJ = 70°C + (0.315)(150) = 117°C

which is near the maximum junction temperature of 125oC.

Note that at higher supply voltages, the junction tempera-

ture is lower due to reduced switch resistance.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC1474/LTC1475. These items are also illustrated

graphically in the layout diagram of Figure 9. Check the

following in your layout:

1. Is the Schottky diode cathode

closely connected to SW

(Pin 5)?

2. Is the 0.1

µF input decoupling capacitor closely con-

nected between VIN (Pin 7) and ground (Pin 4)? This

capacitor carries the high frequency peak currents.

3. When using adjustable version, is the resistive divider

closely connected to the (+) and (–) plates of COUT with

a 10pF capacitor connected across R2?

4. Is the 1000pF decoupling capacitor for the current

sense resistor connected as close as possible to Pins 6

and 7? If no current sense resistor is used, Pins 6 and

7 should be shorted.

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