LTC3035

8

3035f

temperature range. The X5R only loses about 40% of its

rated capacitance over the operating temperature range.

The X5R and X7R dielectrics result in more stable charac-

teristics and are more suitable for use as the output

capacitor. The X7R type has better stability across tem-

perature and bias voltage, while the X5R is less expensive

and is available in higher values. In all cases, the output

capacitance should never drop below 0.4

µF, or instability

or degraded performance may occur.

Charge Pump Component Selection

The flying capacitor controls the strength of the charge

pump. In order for the charge pump to deliver its maximum

available current, a 0.1

µF or greater ceramic capacitor

should be used.

Warning: A polarized capacitor such as

tantalum or aluminum should never be used for the flying

capacitor since its voltage can reverse upon start-up of the

LTC3035. Low ESR ceramic capacitors should always be

used for the flying capacitor.

A 1

µF or greater low ESR (<0.1Ω) ceramic capacitor is

recommended to bypass the BIAS pin. Larger values of

capacitance will not reduce the size of the BIAS ripple

much, but will decrease the ripple frequency proportion-

ally. The BIAS pin should maintain 0.4

µF of capacitance at

all times to ensure correct operation. High ESR tantalum

and electrolytic capacitors may be used, but a low ESR

ceramic must be used in parallel for correct operation. It

is also recommended that IN be bypassed to ground with

a 1

µF or greater ceramic capacitor.

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125

°C). The

power dissipated by the device will be the output current

multiplied by the input/output voltage differential:

The LTC3035 has internal thermal limiting designed to

protect the device during momentary overload conditions.

For continuous normal conditions, the maximum junction

temperature rating of 125

°C must not be exceeded. It is

important to give careful consideration to all sources of

thermal resistance from junction to ambient. Additional

heat sources mounted nearby must also be considered.

For surface mount devices, heat sinking is accomplished

by using the heat-spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through holes can also be used to spread the heat

generated by power devices.

A junction to ambient thermal coefficient of 76

°C/W is

achieved by connecting the Exposed Pad of the DFN

Figure 6. Ceramic Capacitor Temperature Characteristics

Figure 5. Ceramic Capacitor DC Bias Characteristics

DC BIAS VOLTAGE (V)

3035 F05

20

0

–20

–40

–60

–80

–100

0

2

45

6

13

X5R

Y5V

BOTH CAPACITORS ARE 1

µF,

6.3V, 0402 CASE SIZE

TEMPERATURE (

°C)

–50

–100

–80

–60

–40

–20

X5R

Y5V

20

–25

025

50

3035 F06

75

0

BOTH CAPACITORS ARE 1

µF,

6.3V, 0402 CASE SIZE

APPLICATIO S I FOR ATIO