LTC3261

7

3261f

The LTC3261 is a high voltage inverting charge pump. It

supports a wide input power supply range from 4.5V to 32V.

Shutdown Mode

In shutdown mode, all circuitry except the internal bias is

turned off. The LTC3261 is in shutdown when a logic low is

applied to the enable input (EN). The LTC3261 only draws

Constant Frequency Operation

The LTC3261 provides low noise constant frequency opera-

tion when a logic low is applied to the MODE pin. The charge

pump and oscillator circuit are enabled using the EN pin. At

the beginning of a clock cycle, switches S1 and S2 are closed.

cycle, switches S1 and S2 are opened, while switches S3

flying capacitor is grounded and charge is delivered through

The charge transfer frequency can be adjusted between

50kHz and 500kHz using an external resistor on the RT

pin. At slower frequencies the effective open-loop output

to provide smaller average output current. Figure 1 can

be used to determine a suitable value of RT to achieve a

required oscillator frequency. If the RT pin is grounded,

the part operates at a constant frequency of 500kHz.

Burst Mode Operation

The LTC3261 provides low power Burst Mode operation

when a logic high is applied to the MODE pin. In Burst

oscillator to reduce switching losses and goes into a low

current state. This state is referred to as the sleep state in

which the IC consumes only about 60μA. When the output

voltage droops enough to overcome the burst comparator

hysteresis, the part wakes up and commences charge pump

This mode provides lower operating current at the cost of

higher output ripple and is ideal for light load operation.

Thefrequencyofchargingcyclesissetbytheexternalresistor

frequencies.ForBurstModeoperationitisrecommendedthat

the RT pin be tied to GND. This minimizes the charge pump

and optimizes the duration of the low current sleep state.

Figure 1. Oscillator Frequency vs RT

Soft-Start

The LTC3261 has built in soft-start circuitry to prevent

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

achieved by internal circuitry that slowly ramps the amount

of current available at the output storage capacitor. The

soft-start circuitry is reset in the event of a commanded

shutdown or thermal shutdown.

Short-Circuit/Thermal Protection

The LTC3261 has built-in short-circuit current limit as

well as overtemperature protection. During a short-circuit

condition, the part automatically limits its output current

to approximately 160mA. If the junction temperature ex-

ceeds approximately 175°C the thermal shutdown circuitry

disables current delivery to the output. Once the junction

temperature drops back to approximately 165°C current

delivery to the output is resumed. When thermal protection

is active the junction temperature is beyond the specified

operating range. Thermal protection is intended for mo-

mentary overload conditions outside normal operation.

Continuous operation above the specified maximum op-

erating junction temperature may impair device reliability.

RT (kΩ)

1

0

500

600

400

300

200

100

100

1000

10

10000

3261 F01