Rainbow-electronics MAX1721 Manual do Utilizador descarregar pdf (Página 5)

Detailed Description

The MAX1719/MAX1720/MAX1721 capacitive charge

pumps invert the voltage applied to their input. For high-

est 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

shifted downward by V

volts. This connects C1 in par-

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

-V

. The actual voltage at the output is more positive

than -V

, since switches S1–S4 have resistance and the

load drains charge from C2.

MAX1719/MAX1720/MAX1721

SOT23, Switched-Capacitor

Voltage Inverters with Shutdown

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(Circuit of Figure 1, V

= +5V, SHDN = GND (MAX1719), SHDN = IN (MAX1720/MAX1721), C1 = C2 = C3, T

= +25°C, unless

otherwise noted.)

100

150

300

350

250

200

400

0 1.0 1.5 2.0 2.50.5 3.0 3.5 4.0 4.5 5.0

MAX1719/MAX1721

OUTPUT VOLTAGE RIPPLE vs. CAPACITANCE

MAX1720/21toc15

CAPACITANCE (µF)

OUTPUT VOLTAGE RIPPLE (mVp-p)

= +4.75V, V

OUT

= -4.0V

= +3.15V, V

OUT

= -2.5V

= +1.9V, V

OUT

= -1.5V

50µs/div

MAX1721

START-UP FROM SHUTDOWN

= 1kΩ

OUT

2V/div

MAX1720/21toc13

SHDN

5V/div

Noninverting Shutdown Input. Drive this pin low for normal operation; drive it high for

shutdown mode. OUT is actively pulled to ground during shutdown.

SHDN5

GroundGND4

Negative Terminal of Flying CapacitorC1-3

Power-Supply Positive Voltage InputIN2

Inverting Charge-Pump OutputOUT

FUNCTIONNAME

MAX1719/MAX1721

OUTPUT CURRENT vs. CAPACITANCE

MAX1720/21toc14

CAPACITANCE (µF)

OUTPUT CURRENT (mA)

= +1.9V, V

OUT

= -1.5V

= +4.75V, V

OUT

= -4.0V

= +3.15V, V

OUT

= -2.5V

0 1.0 1.5 2.0 2.50.5 3.0 3.5 4.0 4.5 5.0

Positive Terminal of Flying CapacitorC1+6

Pin Description

Inverting Shutdown Input. Drive this pin high for normal operation; drive it low for

shutdown mode. OUT is actively pulled to ground during shutdown.

SHDN

– 5

–

MAX1720

MAX1721

MAX1719

1 2 3 4 5 6 7 8

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