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LTC3230

3230fa

In 2x boost mode as well, the efﬁciency is similar to that

of a linear regulator with an effective input voltage of 2

times the actual input voltage. In an ideal 2x charge pump,

the power efﬁciency would be given by:

η=

LED

•

••

•

Thermal Management

For higher input voltages and maximum output current,

there can be substantial power dissipation in the LTC3230.

If the junction temperature increases above approximately

150°C the thermal shutdown circuitry will automatically

deactivate the output current sources, charge pump and

both LDOs. To reduce maximum junction temperature,

a good thermal connection to the PC board is recom-

mended. Connecting the Exposed Pad to a ground plane

and maintaining a solid ground plane under the device

will reduce the thermal resistance of the package and PC

board considerably.

Its built-in thermal shutdown circuitry will protect the

LTC3230 from short term transient events. For continu-

ous operation the maximum rated junction temperature

is 125°C. The power dissipated by the device is made up

of three components:

1. The LTC3230 IVIN operating current (found in the Elec-

trical Characteristics table) multiplied by VIN.

PQ = IQ • VIN

2. The sum of the LED currents multiplied by the differ-

ence between VIN • Mode and the LED forward voltage

where Mode is 1, 1.5 or 2 depending on the charge

pump mode.

PCP = (VIN • Mode – VLED) • ILEDTOTAL

3. For each LDO, the product of the LDO output current

and the difference between VIN and the LDO.

PLDO = (VIN – VLDO1) • ILDO1 + (VIN – VLDO2) • ILDO2

Given a thermal resistance,

θJA, for the LTC3230 QFN

package of 68°C/W, at an ambient temperature of 70°C

the total power in the LTC3230 should be kept to less than

815mW. Applications in which the LDO output voltages

are set to the lower range and which use a high VIN input

voltage may require limiting the total current output to

keep TJ less than 125°C at the upper ambient temperature

corners.

An example using the parameters in Table 4 shows an ap-

plication that just meets the maximum junction temperature

limit. An increase in VIN, for example, will require reducing

the output current of the charge pump or LDO.

Table 4. TJ Calculation Example Parameters

VIN

3.6V

Mode

1.5x

VLED

3.3V

ILEDTOTAL

100mA (20mA/LED)

VLDO1

1.5V

VLDO2

2.8V

ILDO1

200mA

ILDO2

200mA

θJA

68°C/W

70°C

Total Power Dissipation

799mW

Internal Junction Temperature

124°C

APPLICATIONS INFORMATION

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