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LT3645

36V 500mA Step-Down Regulator and 200mA LDO

Linear Technology 

LT3645

APPLICATIONS INFORMATION

temperature range. The X5R and X7R dielectrics result in

more stable characteristics and are more suitable for use

as the output capacitor. The X7R type has better stability

across temperature, while the X5R is less expensive and

is available in higher values. Care still must be exercised

when using X5R and X7R capacitors; the X5R and X7R

codes only specify operating temperature range and maxi-

mum capacitance change over temperature. Capacitance

change due to DC bias with X5R and X7R capacitors

is better than Y5V and Z5U capacitors, but can still be

significant enough to drop capacitor values below ap-

propriate levels. Capacitor DC bias characteristics tend to

improve as component case size increases, but expected

capacitance at operating voltage should be verified. Voltage

and temperature coefficients are not the only sources of

problems. Some ceramic capacitors have a piezoelectric

response. A piezoelectric device generates voltage across

its terminals due to mechanical stress, similar to the way

a piezoelectric microphone works. For a ceramic capacitor

the stress can be induced by vibrations in the system or

thermal transients.

Precision Undervoltage Lockout

The EN/UVLO pin has an accurate 1.23V threshold that

can be used to shutdown the part when the input voltage

drops below a specified level. To perform this function, a

resistor divider between the EN/UVLO pin and the VIN pin

can be tied as shown in Figure 10. The resistor values can

be determined from the following equation:

R7

"

R8 t

©

ǻ

VIN(MIN)

1.23V

–

1¹»º

With the resistor divider connected, the part will only

operate at input voltages greater than VIN(MIN). Note that

the resistor divider will always draw current from VIN. To

reduce this current, the user might use large value resis-

tors for R7 and R8. This is acceptable as long as R7 and

R8 are selected such that they can supply 10μA to the

EN/UVLO pin. A good value for R8 is 100k.

Output Voltage Sequencing

There are a few applications available for sequencing the

buck and LDO output voltages. In Figures 11 and 12, the

buck output (OUT1) is programmed to 3.3V, while the LDO

output (OUT2) is programmed to 1.8V.

Figure 11 shows a standard configuration where OUT1 and

OUT2 come up as soon as possible. In this configuration,

SW

LT3645

DA

FB

4.7μH

31.6K

10μF

10K

OUT1

VCC2

EN2

OUT2

FB2

12.4k

10k

OUT2

2.2μF

3645 F11

VIN

VIN

LT3645

R7

EN/UVLO

R8

GND

3645 F10

Figure 10. Precision UVLO Circuit

16

EN/UVLO

20V/DIV

OUT1

5V/DIV

OUT2

2V/DIV

NPG

5V/DIV

500μs/DIV

Figure 11. OUT1 and OUT2 Come Up as Soon as Possible

3645f

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