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LTC3210-1

32101fc

Figure 2. Charge Pump Thevenin

Equivalent Open-Loop Circuit

When the LTC3210-1 operates in either 1.5x mode or 2x

mode, the charge pump can be modeled as a Thevenin-

equivalent circuit to determine the amount of current

available from the effective input voltage and effective

open-loop output resistance, ROL (Figure 2).

–

ROL

CPO

1.5VBAT OR 2VBAT

–

32101 F02

ROL is dependent on a number of factors including the

switching term, 1/(2fOSC • CFLY), internal switch resis-

tances and the nonoverlap period of the switching circuit.

However, for a given ROL, the amount of current available

will be directly proportional to the advantage voltage of

1.5VBAT – CPO for 1.5x mode and 2VBAT – CPO for 2x

mode. Consider the example of driving white LEDs from

a 3.1V supply. If the LED forward voltage is 3.8V and the

current sources require 100mV, the advantage voltage for

1.5x mode is 3.1V • 1.5 – 3.8V – 0.1V or 750mV. Notice

that if the input voltage is raised to 3.2V, the advantage

voltage jumps to 900mV— a 20% improvement in avail-

able strength.

Soft-Start

Initially, when the part is in shutdown, a weak switch

connects VBAT to CPO. This allows VBAT to slowly charge

the CPO output capacitor to prevent large charging

currents.

The LTC3210-1 also employs a soft-start feature on its

charge pump to prevent excessive inrush current and

supply droop when switching into the step-up modes. The

current available to the CPO pin is increased linearly over

a typical period of 150μs. Soft-start occurs at the start of

both 1.5x and 2x mode changes.

Charge Pump Strength and Regulation

Regulation is achieved by sensing the voltage at the CPO

pin and modulating the charge pump strength based

on the error signal. The CPO regulation voltages are set

internally, and are dependent on the charge pump modes

as shown in Table 1.

Table 1. Charge Pump Output Regulation Voltages

Charge Pump Mode

Regulated VCPO

1.5x

4.55V

5.05V

OPERATION

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