MAX682 Просмотр технического описания (PDF) - Maxim Integrated

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MAX682

3.3V-Input to Regulated 5V-Output Charge Pumps

Maxim Integrated 

3.3V-Input to Regulated 5V-Output

Charge Pumps

Detailed Description

The MAX682/MAX683/MAX684 charge pumps provide

a regulated 5V output from a 2.7V to 5.5V input. They

deliver a maximum of 250mA, 100mA, or 50mA load

current, respectively. Designed specifically for com-

pact applications, a complete regulator circuit requires

only three small external capacitors and one resistor.

An externally adjustable switching frequency and inno-

vative control scheme allow the circuit to be optimized

for efficiency, size, or output noise. The devices also

contain a shutdown feature.

The MAX682/MAX683/MAX684 consist of an error

amplifier, a 1.23V bandgap reference, an internal resis-

tive feedback network, an oscillator, high-current MOS-

FET switches, and shutdown and control logic (Figure

1). Figure 2 shows an idealized unregulated charge-

pump voltage doubler. The oscillator runs at a 50%

duty cycle. During one half of the period, the transfer

capacitor (CX) charges to the input voltage. During the

other half, the doubler stacks the voltage across CX

and the input voltage, and transfers the sum of the two

voltages to the output filter capacitor (COUT). Rather

than simply doubling the input voltage, the

MAX682/MAX683/MAX684 provide a regulated fixed

output voltage (5V) using either skip mode or constant-

frequency mode. Skip mode and constant-frequency

mode are externally selected via the SKIP input pin.

OUT

IN

IN

S1

CIN

S2

CX

OSC

Figure 2. Unregulated Voltage Doubler

IN

S1

S2

CX

CIN

EN

OSCILLATOR

OUT

COUT

OUT

SKIP

SHDN

1.23V

CONTROL

LOGIC

SHDN

EN

OSC

CXP

SWITCHES

CXN

PGND

Figure 1. Functional Block Diagram

Figure 3. Skip-Mode Regulation

Skip Mode

In skip mode (SKIP = low), the error amplifier disables

switching when it detects an output higher than 5V. The

device then skips switching cycles until the output volt-

age drops. Then the error amplifier reactivates the

oscillator. Figure 3 illustrates the regulation scheme.

This regulation method minimizes operating current

because the device does not switch continuously. SKIP

is a logic input and should not remain floating.

Constant-Frequency Mode

When SKIP is high, the charge pump runs continuously

at the selected frequency. Figure 4 shows a block dia-

gram of the device in constant-frequency mode. The

error amplifier controls the charge on CX by driving the

gate of the N-channel FET. When the output voltage

falls, the gate drive increases, resulting in a larger volt-

age across CX. This regulation scheme minimizes out-

put ripple. Since the device switches continuously, the

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