LTC3230_15 Datasheet, PDF (14/16 Pages) Linear Technology – 5-LED Main/Sub Display Driver with Dual LDO

LTC3230
APPLICATIONS INFORMATION
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 = ^PLED = ^{VLED • ILED} = ^VLED
P_IN V_IN • 2 • I_IN 2 • V_IN
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 I_VIN operating current (found in the Elec-
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
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
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
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
V_IN
3.6V
Mode
1.5x
V_LED
I_LEDTOTAL
V_LDO1
V_LDO2
I_LDO1
I_LDO2
θ_JA
T_A
Total Power Dissipation
3.3V
100mA (20mA/LED)
1.5V
2.8V
200mA
200mA
68°C/W
70°C
799mW
Internal Junction Temperature
124°C
3230fa
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