Rainbow Electronics MAX684 - Manual (Page 7)

MAX682/MAX683/MAX684
3.3V-Input to Regulated 5V-Output
Charge Pumps
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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
to V
IN , the bias current into SHDN determines the
charge-pump frequency. To select the frequency, cal-
culate the external resistor value, R
EXT , using the fol-
lowing formula:
R EXT = 45000 (V IN - 0.69V) / f OSC
where R EXT is in k
Ω
and f OSC is in kHz. Program the
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.
Generally, the transfer capacitor (C X ) will be the small-
est, and the input capacitor (C
IN
) is twice as large as
C X . Higher switching frequencies allow the use of
smaller C X and C IN . The output capacitor (C OUT ) can
be anywhere from 5-times to 50-times larger than C X ,
depending on the mode of operation and ripple toler-
ance. In continuous switching mode, smaller output rip-
ple allows smaller C
OUT . In skip mode, a larger C OUT is
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
C IN
C OUT
C X
OUT
OSC
N-CHANNEL
Figure 4. Constant-Frequency-Mode Regulation
FEATURE
SKIP MODE
(
SKIP
= LOW)
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