Rainbow Electronics MAX684 - Manual (Page 6)

MAX682/MAX683/MAX684
3.3V-Input to Regulated 5V-Output
Charge Pumps
6 _______________________________________________________________________________________
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 (C
X ) charges to the input voltage. During the
other half, the doubler stacks the voltage across C X
and the input voltage, and transfers the sum of the two
voltages to the output filter capacitor (C
OUT ). 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.
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 C
X 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 C
X . This regulation scheme minimizes out-
put ripple. Since the device switches continuously, the
CXP
OUT
1.23V
EN
SHDN
PGND
SWITCHES
CONTROL
LOGIC
CXN
SHDN
SKIP
IN
OSC
IN
S1
S2
C IN C OUT
OUT
C X
OSC
Figure 2. Unregulated Voltage Doubler
IN
S1
S2
C IN
C X
OUT
OSCILLATOR
EN
Figure 3. Skip-Mode Regulation
Figure 1. Functional Block Diagram