LTC3255 datasheet(9/14 Pages) AD

9 / 14 page

LTC3290

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

OPERATION (Refer to the Block Diagram)

Figure 3. Equivalent Open-Loop Circuit

Figure 4. Typical ROL vs Temperature

ply (VAUX) serves as an upper maximum for the offset

voltage. R2 and R1 values must be chosen such that the

offset voltage does not exceed VAUX. For low quiescent

current operation, it is important to note that the external

divider adds a current equal to 2 • (1V/R1) to the total

quiescent current. Recommended range of values for R1

is 100k to 500k.

Soft-Start

The LTC3290 has built in soft-start circuitry to prevent

excessive current flow during start-up. The soft-start is

achievedbyinternalcircuitrythatslowlyrampstheamount

of current available at the output storage capacitors on the

VOUT pin. The soft-start circuitry is reset in the event of a

commanded shutdown or thermal shutdown.

Available Output Current

For the LTC3290, the available output current can be

calculated from the effective open-loop output resistance,

ROL, and the effective output voltage, (VIN + VAUX)(MIN).

From Figure 3, the available current is given by:

IOUT =

VIN + VAUX

(

) – VOUT

ROL

Short-Circuit/Thermal Protection

The LTC3290 has built-in short-circuit current limit as

well as overtemperature protection. During an overcurrent

condition, the part automatically limits output current from

the VOUT pin to 150mA (typ). In the event of a short-circuit

on the VOUT pin, the output current is dialed back to further

reduce power dissipation. If the junction temperature ex-

ceedsapproximately175°Cthethermalshutdowncircuitry

disables current delivery to the output. Once the junction

temperature drops back to approximately 165°C, current

delivery to the output resumes. When thermal protection

is active the junction temperature is beyond the specified

operating range. Thermal protection is intended for mo-

mentary overload conditions outside of normal operation.

Continuous operation above the specified maximum op-

erating junction temperature may impair device reliability.

Input/Output Capacitor Selection

ThestyleandvalueofthecapacitorsusedwiththeLTC3290

determine several important parameters such as output

ripple, charge pump strength and minimum turn-on time.

To reduce noise and ripple, it is recommended that low

ESR ceramic capacitors be used for the charge pump

output. COUT should retain at least 5µF of capacitance over

operating temperature and bias voltage. In Boost Mode,

tantalum and aluminum capacitors can be used in parallel

TEMPERATURE (°C)

–100

100

–50

150

3290 F04

200

VIN = 12V

VAUX = 5V

+–

ROL

IOUT

VOUT

(VIN + VAUX)

3290 F03

–

The value of the ROL depends on many factors such as the

internal oscillator frequency (fOSC), the value of the flying

capacitor (CFLY), the nonoverlap time, the internal switch

resistances (RS) and the ESR of the external capacitors.

Typical ROL values of the boost charge pump as a function

of temperature are shown in Figure 4.

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