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Fabricante

MAX1697
(Rev.:1999)

60mA, SOT23 Inverting Charge Pump with Shutdown

Maxim Integrated 

60mA, SOT23 Inverting Charge Pump

with Shutdown

S1

S2

IN

C1

S3

S4

C2

VOUT = -(VIN)

REQUIV

V+

REQUIV

=

1

fOSC ·

C1

C2

Figure 3b. Equivalent Circuit

VOUT

RL

Figure 2. Ideal Voltage Inverter

fOSC

V+

C1

C2

VOUT

RL

Figure 3a. Switched-Capacitor Model

where the output impedance is roughly approximated

by:

( ) ROUT ≅

1

fOSC

⋅C1 + 2RSW

+ 4ESRC1 + ESRC2

The first term is the effective resistance of an ideal

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

RSW is the sum of the charge pump’s internal switch

resistances (typically 4Ω to 5Ω at VIN = +5V). The typi-

cal output impedance is more accurately determined

from the Typical Operating Characteristics.

Current Limit

The MAX1697 limits its input current upon start-up to

170mA (typ). This prevents low-current or higher output

impedance input supplies (such as alkaline cells) from

being overloaded when power is applied or when the

device awakes from shutdown.

Shutdown

The MAX1697 has a logic-controlled shutdown input.

Driving SHDN low places the device in a low-power

shutdown mode. The charge-pump switching halts,

supply current is reduced to 2nA, and OUT is actively

pulled to ground through a 3Ω resistance.

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.

Thermal Shutdown

The MAX1697 has a thermal shutdown mode for addi-

tional protection against fault conditions. When the tem-

perature of the die exceeds +150°C, the internal clock

stops, suspending the device’s operation. The

MAX1697 resumes operation when the die temperature

falls 15°C. This prevents the device from rapidly oscil-

lating around the temperature trip point.

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:

⋅ ⋅ VRIPPLE

=

IOUT

2(fOSC)C2

+

2

IOUT

ESRC2

Input Bypass Capacitor (C3)

If necessary, bypass the incoming supply to reduce its

AC impedance and the impact of the MAX1697’s switch-

ing noise. A bypass capacitor with a value equal to that

of C1 is recommended.

8 _______________________________________________________________________________________

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