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LTC3230
APPLICATIONS INFORMATION
In 2x boost mode as well, the ef?ciency 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 ef?ciency would be given by:
Given a thermal resistance, θ JA , 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
= =
V IN IN
η=
P LED
P IN
V LED ? I LED
? 2 ? I
V LED
2 ? V IN
are set to the lower range and which use a high V IN input
voltage may require limiting the total current output to
keep T J less than 125°C at the upper ambient temperature
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
corners.
An example using the parameters in Table 4 shows an ap-
plication that just meets the maximum junction temperature
limit. An increase in V IN , for example, will require reducing
the output current of the charge pump or LDO.
Table 4. T J Calculation Example Parameters
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 I VIN operating current (found in the Elec-
V IN
Mode
V LED
I LEDTOTAL
V LDO1
V LDO2
I LDO1
I LDO2
θ JA
T A
Total Power Dissipation
Internal Junction Temperature
3.6V
1.5x
3.3V
100mA (20mA/LED)
1.5V
2.8V
200mA
200mA
68°C/W
70°C
799mW
124°C
trical Characteristics table) multiplied by V IN .
P Q = I Q ? V IN
2. The sum of the LED currents multiplied by the differ-
ence between V IN ? Mode and the LED forward voltage
where Mode is 1, 1.5 or 2 depending on the charge
pump mode.
P CP = (V IN ? Mode – V LED ) ? I LEDTOTAL
3. For each LDO, the product of the LDO output current
and the difference between V IN and the LDO.
P LDO = (V IN – V LDO1 ) ? I LDO1 + (V IN – V LDO2 ) ? I LDO2
3230fa
14
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