3.3V-Input to Regulated 5V-Output

Charge Pumps

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7

output noise contains well-defined frequency compo-

nents, and the circuit requires much smaller external

capacitors for a given output ripple. However, constant-

frequency mode, due to higher operating current, is

less efficient at light loads than skip mode. Note: For

input voltages above 3.6V, the devices must operate in

constant-frequency mode. Table 1 summarizes the

tradeoffs between the two operating modes.

Frequency Selection and Shutdown

The SHDN pin on the MAX682/MAX683/MAX684 per-

forms a dual function: it shuts down the device and

determines the oscillator frequency. The SHDN input

looks like a diode to ground and should be driven

through a resistor.

Driving SHDN low places the device in shutdown

mode. This disables all switches, the oscillator, and

control logic. The device typically draws 0.1µA (5µA

max) of supply current in this mode and the output pre-

sents a 50k

Ω impedance to ground. The device exits

shutdown once SHDN is forward biased (minimum of

1µA of current). The typical no-load shutdown exit time

is 50µs.

When SHDN is pulled high through an external resistor

charge-pump frequency. To select the frequency, cal-

lowing formula:

frequency in the 50kHz to 2MHz range. This frequency

range corresponds to SHDN input currents between

1µA and 50µA. Proper operation of the oscillator is not

guaranteed beyond these limits. Currents lower than

1µA may shut down the device. The forward-biased

diode voltage from the SHDN input to GND has a tem-

perature coefficient of -2mV/°C.

Undervoltage Lockout

The MAX682/MAX683/MAX684 have an undervoltage-

lockout feature that deactivates the devices when the

input voltage falls below 2.25V. Regulation at low input

voltages cannot be maintained. This safety feature

ensures that the device shuts down before the output

voltage falls out of regulation by a considerable amount

(typically 10% with no load). Once deactivated, hys-

teresis holds the device in shutdown until the input volt-

age rises 100mV above the lockout threshold.

Applications Information

Capacitor Selection

The MAX682/MAX683/MAX684 require only three exter-

nal capacitors (Figure 5). Their values are closely linked

to the output current capacity, oscillator frequency, out-

put noise content, and mode of operation.

depending on the mode of operation and ripple toler-

ance. In continuous switching mode, smaller output rip-

required to maintain low output ripple. Tables 2 and 3

show capacitor values recommended for lowest sup-

ply-current operation (skip mode) and smallest size oper-

ation (constant-frequency mode), respectively.

IN

S1

S2

OUT

OSC

N-CHANNEL

Figure 4. Constant-Frequency-Mode Regulation

FEATURE

SKIP MODE

CONSTANT-

FREQUENCY MODE

(SKIP = HIGH)

Best Light-Load

Efficiency

Smallest External

Component Size

Output Ripple

Amplitude and

Frequency

Relatively large

amplitude, variable

frequency

Relatively small

amplitude, constant

frequency

Load Regulation

Very Good

Good

Table 1. Tradeoffs Between Operating

Modes