MAX864EEE-T Maxim Integrated Products, MAX864EEE-T Datasheet - Page 7

• Manufacturer Part Number: MAX864EEE-T
• Description: Charge Pumps Dual-Output Charge Pump
• Manufacturer: Maxim Integrated Products

Specifications of MAX864EEE-T

• Function: Inverting, Step Up
• Output Voltage: - 3.5 V to - 12 V, 3.5 V to 12 V
• Output Current: 20 mA
• Maximum Operating Temperature: + 85 C
• Package / Case: QSOP-16
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT

The MAX864 is not a voltage regulator: the output
source resistance of either charge pump is approxi-
mately 55Ω at room temperature (with V
and V- approach +10V and -10V, respectively, when
lightly loaded. Both V+ and V- will droop toward GND as
the current draw from either V+ or V- increases, since V-
is derived from V+. Treating each converter separately,
the droop of the negative supply (V
uct of the current draw from V- (I
resistance of the negative converter (RS-):
The droop of the positive supply (V
product of the current draw from the positive supply
(I
verter (RS+), where I
and the external load on V+ (I
Determine V+ and V- as follows:
The output resistances for the positive and negative
charge pumps are tested and specified separately. The
positive charge pump is tested with V- unloaded. The
negative charge pump is tested with V+ supplied from
an external source, isolating the negative charge pump.
Current draw from either V+ or V- is supplied by the
reservoir capacitor alone during one half cycle of the
clock. Calculate the resulting ripple voltage on either
output as follows:
where I
ple, with an f
tors, the ripple is 26mV when I
Remember that, in most applications, the total load on
V+ is the V+ load current (I
the negative charge pump (I
LOAD+
V
V
RIPPLE
DROOP+
LOAD
) and the source resistance of the positive con-
V+ = 2V
V - = (V+ - V
=
PUMP
is the load on either V+ or V-. For exam-
= -(2V - V
Dual-Output Charge Pump with Shutdown
= I
1
2
V
DROOP-
I
LOAD
_______________________________________________________________________________________
LOAD+
of 33kHz and 6.8µF reservoir capaci-
IN
IN
LOAD+
- V
(1 / f
DROOP
DROOP+
x RS+ = I
DROOP+
= I
V+
PUMP
V-
Charge-Pump Output
V -
V+
) and the current taken by
is the combination of I
).
):
x RS -
)
) (1 / C
- V
DROOP-
V+
V-
DROOP-
) and the source
+ I
LOAD
IN
DROOP+
RESERVOIR
V -
= 5V); and V+
) is the prod-
)
x RS+
is 12mA.
) is the
)
V-
The MAX864 features a shutdown mode that reduces
the maximum supply current to 1µA over temperature.
The SHDN pin is an active-low TTL logic-level input. If
the shutdown feature is unused, connect SHDN to IN.
In shutdown mode, V+ connects to IN through a 22Ω
switch and V- connects to GND through a 6Ω switch.
Theoretically, a charge-pump voltage multiplier can
approach 100% efficiency under the following condi-
tions:
For the MAX864, the energy loss per clock cycle is the
sum of the energy loss in the positive and negative
converters, as follows:
where V+ and V- are the actual measured output volt-
ages.
The average power loss is simply:
Resulting in an efficiency of:
There will be a substantial voltage difference between
(V+ - V
V+ and V- if the impedances of the pump capacitors
(C1 and C2) are large with respect to their respective
output loads.
Larger reservoir capacitor (C3 and C4) values will
reduce output ripple. Larger values of both pump and
reservoir capacitors will improve efficiency.
_________Efficiency Considerations
The charge-pump switches have virtually no offset,
and extremely low on-resistance.
The drive circuitry consumes minimal power.
The impedances of the reservoir and pump capaci-
tors are negligible.
Total Output Power Total Output Power
LOSS
IN
) and V
CYCLE
P
LOSS
IN
=
= LOSS
for the positive pump, and between
= LOSS
2
1
C1
/
2
1
C
POS
2
V
CYCLE
+ LOSS
V
2
x f
2
2
PUMP
V
NEG
V
Shutdown
V
2
IN
P
LOSS
7