Max865 compact, dual-output charge pump – Rainbow Electronics MAX865 User Manual
Page 6
converter (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 resistance 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 the typi-
cal f
PUMP
of 30kHz with 3.3µF reservoir capacitors, the
ripple is 25mV when I
LOAD
is 5mA. 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-
).
Efficiency Considerations
Theoretically, a charge-pump voltage multiplier can
approach 100% power efficiency under the following
conditions:
•
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 MAX865, the energy loss per clock cycle is the
sum of the energy loss in the positive and negative
converters, as follows:
The average power loss is simply:
Resulting in an efficiency of:
MAX865
C1+
1
2
3
4
C1-
GND
3.3
µ
F
3.3
µ
F
3.3
µ
F
3.3
µ
F
3.3
µ
F
3.3
µ
F
OUT+
IN
OUT-
8
7
6
5
C2-
C2-
V-
V+
IN
GND
MAX865
C1+
1
2
3
4
C1-
8
7
6
5
C2+
C2-
V
IN
V-
V+
IN
GND
Figure 3. Paralleling MAX865s
MAX865
Compact, Dual-Output Charge Pump
6
_______________________________________________________________________________________
V
= I
x RS+ = I
+ I
x RS+
DROOP+
LOAD+
V+
V -
(
)
P
= LOSS
x f
LOSS
CYCLE
PUMP
η =
−
(
)
Total Output Power Total Output Power
P
LOSS
/
V
= I
(1 / f
) (1 / C
)
RIPPLE
1
2
LOAD
PUMP
RESERVOIR
V+ = 2V - V
V - = (V+ - V
) = -(2V
- V
- V
)
IN
DROOP+
DROOP
IN
DROOP+
DROOP -
LOSS
= LOSS
+ LOSS
=
C1
CYCLE
POS
NEG
1
2
1
2
V
V
V
C
V
V
IN
+
( )
−
+
( ) ( )
+
+
( )
−
−
( )
2
2
2
2
2