TLE2662IDW datasheet(35/40 Pages) TI

35 / 40 page

TLE2662

DUAL

µPOWER JFET-INPUT OPERATIONAL AMPLIFIER

WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER

SLOS118B – DECEMBER 1992 – REVISED AUGUST 1994

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

regulation (continued)

The functional block diagram shows that the maximum regulated output voltage is limited by the supply voltage.

For the basic configuration,

SCOUT  referenced to GND of the TLE2662 must be less than the total of the

supply voltage minus the voltage loss due to the switches. The voltage loss versus output current due to the

switches can be found in the typical performance curves.

capacitor selection

While the exact values of CIN and COUT are noncritical, good-quality low-ESR capacitors such as solid

tantalum are necessary to minimize voltage losses at high currents. For CIN, the effect of the equivalent series

resistance (ESR) of the capacitor is multiplied by four, since switch currents are approximately two times higher

than output current. Losses occur on both the charge and discharge cycle, which means that a capacitor with

Ω of ESR for CIN has the same effect as increasing the output impedance of the switched-capacitor section

by 4

Ω. This represents a significant increase in the voltage losses. COUT is alternately charged and discharged

at a current approximately equal to the output current. The ESR of the capacitor causes a step function to occur

in the output ripple at the switch transitions. This step function degrades the output regulation for changes in

output load current and should be avoided. A technique used is to parallel a smaller tantalum capacitor with a

large aluminum electrolytic capacitor to gain both low ESR and reasonable cost.

output ripple

The peak-to-peak output ripple is determined by the output capacitor and the output current values.

Peak-to-peak output ripple is approximated as shown in equation 4:

DV +

2fC

(4)

where:

∆V = peak-to-peak ripple

fOSC = oscillator frequency

For output capacitors with significant ESR, a second term must be added to account for the voltage step at the

switch transitions. This step is approximately equal to equation 5:

ESR of C

(5)

power dissipation (switched-capacitor section only)

The power dissipation of any TLE2662 circuit must be limited so that the junction temperature of the device does

not exceed the maximum junction temperature ratings. The total power dissipation is calculated from two

components, the power loss due to voltage drops in the switches, and the power loss due to drive current losses.

The total power dissipated by the TLE2662 is calculated as shown in equation 6:

[ (V

–

O )

)(I

) (0.2)

(6)

where both VCC and SCOUT refer to GND. The power dissipation is equivalent to that of a linear regulator. Due

to limitations of the DW package, steps must be taken to dissipate power externally for large input or output

differentials. This is accomplished by placing a resistor in series with CIN as shown in Figure 64. A portion of

the input voltage is dropped across this resistor without affecting the output regulation. Since switch current is

approximately 2.2 times the output current and the resistor causes a voltage drop when CIN is both charging

and discharging, the resistor chosen is as shown in equation 7.

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