lm2780tp National Semiconductor Corporation, lm2780tp Datasheet - Page 5

• Manufacturer Part Number: lm2780tp
• Description: Ultra-low Ripple Switched Capacitor Voltage Inverter
• Manufacturer: National Semiconductor Corporation

Block Diagram
Operation Description
The LM2780 is a switched capacitor voltage inverter that
produces a negative output voltage from a positive input
voltage. To minimize output voltage ripple, the LM2780 con-
tains two charge pump inverters that operate 180˚ out of
phase. These two charge pumps can be seen in the block
diagram above.
A basic switched capacitor inverter is a two-phase charge
pump circuit that transfers charge from the input to the
output using a flying capacitor. Charge Pump 1 in the block
diagram will be used to illustrate the operation of a single
charge pump. Disregard Charge Pump 2 in the diagram for
now. In phase 1 (φ1), the CLK1A signal of Charge Pump 1 is
high, and MOSFET switches S1A and S1B are ON. For
Charge Pump 1, this is the charge phase, as the flying
capacitor C1 is charged to the input voltage. In the second
phase (φ2), the positive-charged plate of C1 is connected to
GND through switch S1C, and the negative-charged plate is
connected to the output through switch S1D. This results in
the output voltage being approximately equal in magnitude
to the input voltage, but opposite in sign (V
this pump phase, charge stored on the flying capacitor sup-
plies the current to the load and transfers charge to the
output capacitor.
This basic inverter has ripple in the output voltage. During
the charge phase, the load current is supplied by the charge
on the output capacitor. As charge is pulled off C
voltage across the capacitor drops, and the magnitude of the
output voltage ( |V
OUT
| ) falls. During the pump phase, the
OUT
~ = - V
OUT
IN
, the
). In
5
flying capacitor transfers charge to the C
the voltage across the C
voltage.
The LM2780 contains two charge pump inverters running
180˚ out of phase. Notice in the block diagram that the clock
connections on Charge Pump 2 are opposite of the clock
connections to Charge Pump 1. Thus, when Charge Pump 1
is in the charge phase, Charge Pump 2 is in the pump phase,
and vice-versa. With this double-pump technology, there is
always a flying capacitor connected to the output. This re-
sults in significantly lower output voltage ripple than that of a
single-pump inverter.
OUTPUT VOLTAGE RIPPLE
Several factors influence the magnitude of voltage ripple on
the output of the LM2780. Primary influences are output
capacitance, flying capacitance, load current, input voltage,
and output capacitor ESR.
The magnitude of the output voltage ripple is difficult to
predict with equations when the LM2780 is double pumping.
Refer to the Typical Performance Characteristic curves for
this information. In general, ripple decreases with increased
output capacitance, decreased load current, a lower input
voltage, a smaller flying capacitance and/or a smaller output
capacitor ESR.
When only one charge pump is active (C2 removed), the
output ripple is more predictable, and can be estimated with
the following equation:
V
RIPPLE
= [(0.5xI
OUT
) / (f
OUT
SW
xC
and the magnitude of the output
OUT
) ] + (2xI
20044610
OUT
OUT
, elevating both
xESR
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Cout
)