SP6828 Datasheet, PDF (10/17 Pages) Sipex Corporation – +3V Low Power Voltage Inverters

DESCRIPTION
The SP6828/6829 devices are CMOS Charge
Pump Voltage Converters that can be used to
invert a +1.15V to +4.2V input voltage. These
devices are ideal for designs involving battery-
powered and/or board level voltage conversion
applications.
The typical operating frequency of the SP6828
is 12kHz. The typical operating frequency of the
SP6829 is 35kHz. The SP6828 has a typical
operating current of 20µA and the SP6829
operates at 40µA. Both devices can output 25mA
with a voltage drop of 500mV. The devices are
ideal for designs using +3.3V or +3.6V lithium
ion batteries such as cell phones, PDAs, medical
instruments, and other portable equipment. The
SP6828/6829 devices combine a high efficiency
with a low quiescent current.
THEORY OF OPERATION
The SP6828/6829 devices should theoretically
produce an inverted input voltage. In real world
applications, there are small voltage drops at the
output that reduce efficiency. The circuit of an
ideal voltage inverter can be found in Figure 21.
The voltage inverters require two external
capacitors to store the charge. A description of
the two phases follows:
Phase 1
In the first phase of the clock cycle, switches S2
and S4 are opened and S1 and S3 closed. This
connects the flying capacitor, C1, from V_IN to
ground. C1 charges up to the input voltage applied
at V_IN.
Phase 2
In the second phase of the clock cycle, switches
S2 and S4 are closed and S1 and S3 are opened.
This connects the flying capacitor, C1, in parallel
with the output capacitor, C2. The charge stored
in C1 is now transferred to C2. Simultaneously,
the negative side of C2 is connected to V and
OUT
the positive side is connected to ground. With
the voltage across C2 smaller than the voltage
across C1, the charge flows from C1 to C2 until
the voltage at the V equals -V .
OUT
IN
V_OUT = -VIN
V_IN
S1
S2
C1
C2
S3
S4
V_OUT
Figure 21. Circuit for an Ideal Voltage Inverter
Charge-Pump Output
The output of the SP6828/6829 devices is not
regulated and therefore is dependent on the
output resistance and the amount of load current.
As the load current increases, losses may slightly
increase at the output and the voltage may become
slightly more positive. The loss at the negative
output, V_LOSS, equals the current draw, I_OUT, from
V_OUT times the negative converter's source
resistance, R :
S
V_LOSS = I_OUT x R_S.
The actual inverted output voltage at V will
OUT
equal the inverted voltage difference of V and
IN
V_LOSS:
V_OUT = -(V_IN - V_LOSS).
Efficiency
Theoretically, the total power loss of a switched
capacitor voltage converter can be summed up as
follows:
∑P_LOSS
=
P
INT
+
P
CAP
+
P,
CONV
where P_LOSS is the total power loss, P_INT is the total
internal loss in the IC including any losses in the
MOSFET switches, P is the resistive loss of
CAP
SP6828DS/11
SP6828/6829 +3V Low Power Voltage Inverter
10
© Copyright 2000 Sipex Corporation