Detailed Description

The MAX1852/MAX1853 charge pumps invert the volt-

age applied to their input. For highest performance use

low equivalent series resistance (ESR) capacitors (e.g.,

ceramic).

During the first half-cycle, switches S2 and S4 open,

switches S1 and S3 close, and capacitor C1 charges to

the voltage at IN (Figure 2). During the second half-

cycle, S1 and S3 open, S2 and S4 close, and C1 is level

allel with the reservoir capacitor C2. If the voltage across

C2 is smaller than the voltage across C1, charge flows

from C1 to C2 until the voltage across C2 reaches

load drains charge from C2.

Efficiency Considerations

The efficiency of the MAX1852/MAX1853 is dominated

output current; it is given by:

where the output impedance is roughly approximated

by:

The first term is the effective resistance of an ideal

switched-capacitor circuit (Figures 3a and 3b), and

resistances (typically 6

put impedance is more accurately determined from the

Typical Operating Characteristics.

Shutdown

The MAX1852/MAX1853 have a logic-controlled shut-

down input. Driving SHDN low places the devices in a

low-power shutdown mode. The charge-pump switch-

ing halts, supply current is reduced to 2nA.

Driving SHDN high will restart the charge pump. The

switching frequency and capacitor values determine how

soon the device will reach 90% of the input voltage.

Applications Information

Capacitor Selection

The charge-pump output resistance is a function of the

ESR of C1 and C2. To maintain the lowest output resis-

tance, use capacitors with low ESR. (See Table 1 for a

list of recommended manufacturers.) Tables 2 and 3

suggest capacitor values for minimizing output resis-

tance or capacitor size.

Flying Capacitor (C1)

Increasing the flying capacitor’s value reduces the out-

put resistance. Above a certain point, increasing C1’s

capacitance has negligible effect because the output

resistance is then dominated by internal switch resis-

tance and capacitor ESR.

Output Capacitor (C2)

Increasing the output capacitor’s value reduces the

output ripple voltage. Decreasing its ESR reduces both

output resistance and ripple. Lower capacitance values

can be used with light loads if higher output ripple can

be tolerated. Use the following equation to calculate the

peak-to-peak ripple:

R

1

f

x C1

2R

4ESR

ESR

OUT

OSC

SW

C1

C2

+

I

II

1

I

x R

V

OUT

OUT

Q

OUT

OUT

IN

η≅

+

−













SC70 Inverting Charge Pumps

with Shutdown

_______________________________________________________________________________________

5

Pin Description

6

Positive Terminal of the Flying

Capacitor

1

Inverting Charge-Pump Output

2

Ground

3

Shutdown Input. Drive this pin high

for normal operation; drive it low for

shutdown mode.

4

Power-Supply Voltage Input. Input

range is +2.5V to +5.5V.

5

Negative Terminal of the Flying

Capacitor

PIN

FUNCTION

NAME

C1+

OUT

GND

SHDN

IN

C1-

TE: (

C1

C2

41

3

ON

OFF

5

6

2

C3

C1+

C1-

IN

SHDN

OUT

GND

INPUT

2.5V TO 5.5V

NEGATIVE

OUTPUT

MAX1852

MAX1853

Figure 1. Typical Application Circuit