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LTC3200

Low Noise, Regulated Charge Pump DC/DC Converters

Linear Technology 

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OPERATIO

Maximum Available Output Current

For the adjustable LTC3200, the maximum available out-

put current and voltage can be calculated from the effec-

tive open-loop output resistance, ROL, and effective output

voltage, 2VIN(MIN).

ROL

–+ 2VIN

+

IOUT

VOUT

–

32005 F02

Figure 2. Equivalent Open-Loop Circuit

From Figure 2 the available current is given by:

IOUT

=

2VIN – VOUT

ROL

Typical ROL values as a function of temperature are shown

in Figure 3.

11

IOUT = 100mA

CFLY = 1µF

VFB = 0V

10

VIN = 2.7V

VIN = 3.3V

9

8

–50 –25 0

25 50

75 100

AMBIENT TEMPERATURE (°C)

32005 • F03

Figure 3. Typical ROL vs Temperature

VIN, VOUT Capacitor Selection

The style and value of capacitors used with the LTC3200/

LTC3200-5 determine several important parameters such

as regulator control loop stability, output ripple, charge

pump strength and minimum start-up time.

To reduce noise and ripple, it is recommended that low

ESR (< 0.1Ω) ceramic capacitors be used for both CIN

and COUT. These capacitors should be 0.47µF or greater.

LTC3200/LTC3200-5

Tantalum and aluminum capacitors are not recommended

because of their high ESR.

The value of COUT directly controls the amount of output

ripple for a given load current. Increasing the size of COUT

will reduce the output ripple at the expense of higher

minimum turn on time and higher start-up current. The

peak-to-peak output ripple is approximately given by the

expression:

VRIPPLEP−P

≅

IOUT

2fOSC• COUT

Where fOSC is the LTC3200/LTC3200-5’s oscillator fre-

quency (typically 2MHz) and COUT is the output charge

storage capacitor.

Both the style and value of the output capacitor can signifi-

cantly affect the stability of the LTC3200/LTC3200-5. As

shown in the Block Diagrams, the LTC3200/LTC3200-5

use a linear control loop to adjust the strength of the charge

pump to match the current required at the output. The

error signal of this loop is stored directly on the output

charge storage capacitor. The charge storage capacitor

also serves to form the dominant pole for the control loop.

To prevent ringing or instability on the LTC3200-5 it is

important for the output capacitor to maintain at least 0.47µF

of capacitance over all conditions. On the adjustable

LTC3200 the output capacitor should be at least 0.47µF ×

5V/VOUT to account for the alternate gain factor.

Likewise excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3200/LTC3200-5.

The closed loop output resistance of the LTC3200-5 is

designed to be 0.5Ω. For a 100mA load current change,

the output voltage will change by about 50mV. If the output

capacitor has 0.3Ω or more of ESR, the closed loop

frequency response will cease to roll off in a simple one

pole fashion and poor load transient response or instabil-

ity could result. Ceramic capacitors typically have excep-

tional ESR performance and combined with a tight board

layout should yield very good stability and load transient

performance.

As the value of COUT controls the amount of output

ripple, the value of CIN controls the amount of ripple

present at the input pin (VIN). The input current to the

7

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