LM828 datasheet(5/15 Pages) TI

5 / 15 page

LM828

www.ti.com

SNOS035D – MARCH 2010 – REVISED MAY 2013

Pin Functions

PIN DESCRIPTIONS

Name

Function

OUT

Negative voltage output.

V+

Power supply positive input.

CAP

−

Connect this pin to the negative terminal of the charge-pump capacitor.

GND

Power supply ground input.

CAP+

Connect this pin to the positive terminal of the charge-pump capacitor.

Circuit Description

The LM828 contains four large CMOS switches which are switched in a sequence to invert the input supply

voltage. Energy transfer and storage are provided by external capacitors. Figure 12 illustrates the voltage

conversion scheme. When S1 and S3 are closed, C1 charges to the supply voltage V+. During this time interval,

switches S2 and S4 are open. In the second time interval, S1 and S3 are open; at the same time, S2 and S4 are

closed, C1 is charging C2. After a number of cycles, the voltage across C2 will be pumped to V+. Since the anode

of C2 is connected to ground, the output at the cathode of C2 equals −(V+) when there is no load current. The

output voltage drop when a load is added is determined by the parasitic resistance (Rds(on) of the MOSFET

switches and the ESR of the capacitors) and the charge transfer loss between capacitors.

Figure 12. Voltage Inverting Principle

Application Information

SIMPLE NEGATIVE VOLTAGE CONVERTER

The main application of LM828 is to generate a negative supply voltage. The voltage inverter circuit uses only

two external capacitors as shown in the Basic Application Circuits. The range of the input supply voltage is 1.8V

to 5.5V.

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a

resistance. The voltage source equals

−(V+). The output resistance, Rout , is a function of the ON resistance of

the internal MOSFET switches, the oscillator frequency, the capacitance and the ESR of both C1 and C2. Since

the switching current charging and discharging C1 is approximately twice as the output current, the effect of the

ESR of the pumping capacitor C1 will be multiplied by four in the output resistance. The output capacitor C2 is

charging and discharging at a current approximately equal to the output current, therefore, this ESR term only

counts once in the output resistance. A good approximation of Rout is:

(1)

where RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 12.

High capacitance, low ESR capacitors will reduce the output resistance.

The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the

output capacitor C2:

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