Max864 dual-output charge pump with shutdown, Efficiency considerations – Rainbow Electronics MAX864 User Manual
Page 7
MAX864
Dual-Output Charge Pump with Shutdown
_______________________________________________________________________________________
7
Charge-Pump Output
The MAX864 is not a voltage regulator: the output
source resistance of either charge pump is approxi-
mately 55
Ω
at room temperature (with V
IN
= 5V); and 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
DROOP-
) is the prod-
uct of the current draw from V- (I
V-
) and the source
resistance of the negative converter (RS-):
The droop of the positive supply (V
DROOP+
) is the
product of the current draw from the positive supply
(I
LOAD+
) and the source resistance of the positive con-
verter (RS+), where I
LOAD+
is the combination of I
V-
and the external load on V+ (I
V+
):
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
LOAD
is the load on either V+ or V-. For exam-
ple, with an f
PUMP
of 33kHz and 6.8µF reservoir capaci-
tors, the ripple is 26mV when I
LOAD
is 12mA.
Remember that, in most applications, the total load on
V+ is the V+ load current (I
V+
) and the current taken by
the negative charge pump (I
V-
).
Shutdown
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 22
Ω
switch and V- connects to GND through a 6
Ω
switch.
_________Efficiency Considerations
Theoretically, a charge-pump voltage multiplier can
approach 100% efficiency under the following condi-
tions:
•
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.
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
IN
) and V
IN
for the positive pump, and between
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.
V
= I
x RS -
DROOP -
V -
V
= I
x RS+ = I
+ I
x RS+
DROOP+
LOAD+
V+
V -
(
)
V+ = 2V - V
V - = (V+ - V
)
= -(2V
- V
- V
)
IN
DROOP+
DROOP
IN
DROOP+
DROOP -
V
= I
(1 / f
) (1 / C
)
RIPPLE
1
2
LOAD
PUMP
RESERVOIR
LOSS
= LOSS
+ LOSS
=
C1
CYCLE
POS
NEG
1
2
1
2
V
V
V
C
V
V
IN
+
( )
−
+
( ) ( )
+
+
( )
−
−
( )
2
2
2
2
2
P
= LOSS
x f
LOSS
CYCLE
PUMP
η =
−
(
)
Total Output Power Total Output Power
P
LOSS
/