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LT3645

3645f

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

VIN(MIN)

1.23V

–1

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,

APPLICATIONS INFORMATION

Figure 10. Precision UVLO Circuit

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

3645 F10

GND

VIN

EN/UVLO

LT3645

3645 F11

4.7μH

31.6K

OUT1

OUT2

10μF

10K

VCC2

EN2

OUT2

FB2

EN/UVLO

20V/DIV

OUT1

5V/DIV

OUT2

2V/DIV

NPG

5V/DIV

500μs/DIV

12.4k

2.2μF

10k

LT3645

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