sp784ct Exar Corporation, sp784ct Datasheet - Page 10

• Manufacturer Part Number: sp784ct
• Description: Programmable Charge Pump Corporation
• Manufacturer: Exar Corporation

Figure 4. Charge Pump Phase 2 for 10V.
Figure 6. Charge Pump Phase 3.
The oscillator frequency or clock rate for
the charge pump is designed for low power
operation. The oscillator changes from a high
frequency mode (400kHz) to a low frequency
mode (20kHz) when the SD pin goes to a logic
"1". The lower frequency allows the SP782/
SP784 to conserve power when the outputs are
not being used.
EFFICIENCY INFORMATION
A charge pump theoretically produces a doubled
voltage at 100% efficiency. However in the real
world, there is a small voltage drop on the output
which reduces the output efficiency. The SP782
and SP784 can usually run 99.9% efficient with-
out driving a load. While driving a 1k load, the
SP782 and SP784 remain at least 90% efficient.
The efficiency changes as the external charge
pump capacitors are varied. Larger capacitor
values will strengthen the output and reduce
output ripple usually found in all charge pumps.
Although smaller capacitors will cost less and
SP782/SP784 DS/08
Total Output Voltage Efficiency =
[(V
C
C
OUT
1
1
+
–
+
–
V
V
V
V
Power Loss = I
V
V
–5V
CC
+) / (2*V
CC
OUT
OUT
DROP
DROP
= +5V
= +5V
+ = 2*V
– = –2*V
– = (I–)*(R
+ = (I+)*(R
–10V
C
C
CC
2
2
+5V
+
–
+
)] + [(V
–
–5V
CC
CC
OUT
+ V
+ V
OUT
OUT
*(V
DROP
OUT
–)
DROP
+)
DROP
+
+
–
–
C
C
–) / (–2*V
+
C
C
4
4
–
3
3
–
+
–
+
)
V
V
V
V
DD
DD
SS
SS
Storage Capacitor
Storage Capacitor
Storage Capacitor
Storage Capacitor
CC
SP782/784 Programmable Charge Pump
)] ;
10
Figure 5. Charge Pump Phase 2 for 5V.
Figure 7. Charge Pump Phase 4.
save board space, lower values will reduce the
output drive capability.
The output voltage ripple is also affected by the
capacitors, specifically C3 and C4. Larger val-
ues will reduce the output ripple for a given load
of current. The current drawn from either output
is supplied by just the storage capacitor, C3 or
C4, during one half cycle of the internal oscilla-
tor. Note that the output current from the postive
charge pump is the load current plus the current
taken by the negative charge pump. Thus the
formula representation for the output ripple
voltage is:
To minimize the output ripple, the C3 and C4
storage capacitors can be increased to over 10 F
whereas the pump capacitors can range from
1 F to 5 F.
Multiple SP782/784 charge pumps can be
connected in parallel. However, the output
resistance on both pump outputs will be
reduced. The effective output resistance is the
output resistance of one pump divided by the
number of charge pumps connected. It is
important to keep the C1 and C2 capacitors
separate for each charge pump. The storage
capacitors, C3 and C4, can be shared.
V
V
RIPPLE
RIPPLE
C
C
1
1
+
–
+
–
V
V
+ = {1 / (f
– = {1 / (f
CC
CC
= +5V
= +5V
C
C
OSC
OSC
–5V
2
2
+10V
+
+
–
–
) * 1 / C3} * 0.5 * I
) * 1 / C3} * 0.5 * I
© Copyright 2000 Sipex Corporation
+
+
–
–
C
C
C
C
4
4
–
3
–
3
+
+
V
V
V
V
DD
DD
SS
SS
Storage Capacitor
Storage Capacitor
Storage Capacitor
Storage Capacitor
OUT
OUT
–
+