High-Frequency, Regulated,

200mA, Inverting Charge Pump

_______________________________________________________________________________________

7

Adjust the output voltage to a negative voltage from

shown in Figures 1 and 4. FB servos to GND. Choose

R1 to be 100k

Ω or less. Calculate R2 for the desired

output voltage:

source.

Typically, choose a voltage-divider current of at least

30µA to minimize the effect of FB input current and

capacitance:

R1

Capacitor Selection

The appropriate capacitors used with the MAX889

depend on the switching frequency. Table 1 provides

Surface-mount ceramic capacitors are preferred for

and low ESR. To ensure proper operation over the

entire temperature range, choose ceramic capacitors

with X7R (or equivalent) low-temperature-coefficient

(tempco) dielectrics. See Table 2 for a list of suggested

capacitor suppliers.

The output capacitor stores the charge transferred from

the flying capacitor and services the load between

oscillator cycles. A good general rule is to make the

output capacitance at least five-times greater than the

flying capacitor.

Choosing a low-ESR capacitor of sufficient value is impor-

tant in minimizing the peak-to-peak output voltage ripple,

which is approximated by the following equation:

switching frequency. Ceramic capacitors have the lowest

capacitance at low cost is desired, a low-ESR tantalum

suggested capacitor suppliers.

To ensure stability over the entire operating temperature

range, choose a low-ESR output capacitor using the fol-

lowing equation:

minimum oscillator frequency in the Electrical

Characteristics table.

To ensure stability for regulated output mode, suitable

output capacitor ESR should be determined by the follow-

ing equation:

Power Dissipation

The power dissipated in the MAX889 depends on the

input voltage, output voltage, and output current. Device

power dissipation is accurately described by:

less than the package dissipation rating (see Absolute

Maximum Ratings). Pay particular attention to power dis-

sipation limits when generating small negative voltages

from large positive input voltages.

Layout Considerations

The MAX889’s high oscillator frequencies demand

good layout techniques that ensure stability and help

maintain the output voltage under heavy loads. Take

the following steps to ensure optimum layout:

1) Mount all components as close together as possible.

2) Place the feedback resistors R1 and R2 close to the

FB pin, and minimize the PC trace length at the FB

circuit node.

3) Keep traces short to minimize parasitic inductance

and capacitance.

star ground configuration (see the MAX889SEVKIT

layout).

R

19.2 x 10

I

R2

R1

ESR

-3

OUT

≤













+









1

C

15.5

f

R1

R1 + R2

I

OUT

MIN

OUT

≥

















V

=

I

2 x f

C

2 x I

ESR

RIPPLE

OUT

OSC

OUT

OUT

COUT

+