LTC3203B datasheet(11/16 Pages) LINER

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LTC3203/LTC3203-1

LTC3203B/LTC3203B-1

32031fa

resistor RX. Thus, this configuration will have the highest

loop gain giving it the lowest closed-loop output resis-

tance. Likewise it will also require the largest amount of

output capacitance to preserve stability.

Effective Open Loop Output Resistance (ROL)

The effective open loop output resistance (ROL) of a

charge pump is a very important parameter, which deter-

mines its strength. The value of this parameter depends on

many factors such as the oscillator frequency (fOSC), the

value of the flying capacitor (CFLY), the non-overlap time,

the internal switch resistances (RS), and the ESR of the

external capacitors.

Maximum Available Output Current

Figure 3 shows how the LTC3203/LTC3203-1/LTC3203B/

LTC3203B-1 can be modeled as a Thevenin-equivalent

circuit.

Thus the maximum available output current and voltage

can be calculated from the effective open-loop output

resistance, ROL, and the effective output voltage, 1.5VIN

(in 1.5x mode) or 2VIN (in 2x mode). From Figure 3, the

available current is given by:

OUT

−

.mod

In x

OUT

−

2mod

As evident from the above two equations, with the same

VIN and ROL, the 2x mode will give more output current

than the 1.5x mode.

Programming the MODE Pin

By connecting a resistor divider to the MODE pin, the VIN

voltage at which the chip switches modes can be accu-

rately programmed.

When VIN ramps up, the voltage at the MODE pin crosses

VMODEH and the chip switches from 2x mode to 1.5x mode.

When VIN starts to drop, the voltage at the MODE pin

crosses VMODEL and the chip switches back to 2x mode.

The MODE pin resistor ratio must be selected such that at

the switch point the output is still able to maintain regula-

tion at maximum IOUT:

1.5 • VIN(1.5x) – VOUT > IOUT • ROL(1.5X)

The minimum VIN operating in 1.5x mode occurs at the

switch point where:

MODEL

MODE

⎛

⎝⎜

⎞

⎠⎟

•

therefore:

.•

•

)(

MODEL

MODE

X M

⎛

⎝⎜

⎞

⎠⎟

OUT MAX

OUT MIN

MODE

OUT M

)(

)

(

)

(

•

IIN

x MAX

OUT MAX

MODEL

)

( .

)()

()

•

.•

–

Figure 3. Charge Pump Open-Loop Thevenin-Equivalent Circuit

3203 F03

–

1.5VIN

2VIN

ROL

VOUT

–

APPLICATIO S I FOR ATIO

3203 F04

LTC3203/

LTC3203B

CIN

VIN

MODE

GND

RMODE1

RMODE2

9, 11

Figure 4

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