Linear Mode (Constant-Frequency Mode)

In LIN mode (LIN/SKIP = IN), the charge pump runs con-

tinuously at 350kHz. The MAX1673 controls the charge

to increased gate drive. Since the device switches con-

tinuously, the regulation scheme minimizes output ripple,

the output noise contains well-defined frequency compo-

nents, and the circuit requires much smaller external

capacitors than in Skip mode for a given output ripple.*

However, LIN mode is less efficient than Skip mode due

to higher operating current (8mA typical).

Skip Mode

In Skip mode (LIN/SKIP = GND), the device switches

only as needed to maintain regulation on FB. Switching

cycles are skipped until the voltage on FB rises above

GND. Skip mode has higher output noise than LIN

mode, but minimizes operating current.

Shutdown Mode

When SHDN (a CMOS-compatible input) is driven low,

the MAX1673 enters low-power shutdown mode.

Charge-pump switching action halts and an internal 1

Ω

drive high for normal operation.

*See Output Ripple vs. Load Current in

Typical Operating Characteristics.

Applications Information

Resistor Selection

(Output Voltage Selection)

voltage biasing the voltage-divider network (R1, R2).

ed voltage or if greater accuracy is desired (Figure 4).

R1 and R2 as shown in Figures 1 and 4. In either regu-

lating mode (LIN or Skip), FB servos to 0V. Use the

following equations to select R1 and R2 for the desired

output voltage:

reference source.

Typically, choose a voltage-divider current of 50µA to

minimize the effect of FB input current:

Capacitor Selection

specified load current. However, for minimum ripple in

Skip mode, this value may need to be increased.

Maxim recommends 2.2µF.

Surface-mount ceramic capacitors are preferred for

lent series resistance (ESR). To ensure proper opera-

tion over the entire temperature range, choose ceramic

capacitors with X7R (or equivalent) low-temperature-

coefficient (tempco) dielectrics. See Table 1 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 ten times greater than that

of the flying capacitor.

When in Skip mode, output ripple depends mostly on

two parameters: charge transfer between the capaci-

The charging current creates a negative voltage pulse

Secondly, the ripple contribution due to charge transfer

between capacitors creates a pulse as charge flows to

to-peak ripple because their peaks do not occur at the

same time. It is best to use only the dominant term. The

expression for the ripple component predominantly due

V

= - V

R2

R1

OUT

REF

Regulated, 125mA-Output,

Charge-Pump DC-DC Inverter

_______________________________________________________________________________________

7

MAX1673

IN

INPUT

5.0V

OUTPUT

-3V

10

µF

22

µF

2.2

µF

LIN/SKIP

FB

R1

100k

R2

60.4k

OUT

GND

CAP+

ON

OFF

4

2

3

SHDN

CAP-

LIN

SKIP

1

7

5

6

8

5V

R2

R1