LM828 datasheet(5/8 Pages) NSC

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Pin Description

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 capaci-

tors.

Figure 2 illustrates the voltage conversion scheme.

When S

1 and S3 are closed, C1 charges to the supply volt-

age V+. During this time interval, switches S

2 and S4 are

open. In the second time interval, S

1 and S3 are open; at the

same time, S

2 and S4 are closed, C1 is charging C2. After a

number of cycles, the voltage across C

2 will be pumped to

V+. Since the anode of C

2 is connected to ground, the output

at the cathode of C

2 equals −(V+) when there is no load cur-

rent. The output voltage drop when a load is added is deter-

mined by the parasitic resistance (R

ds(on) of the MOSFET

switches and the ESR of the capacitors) and the charge

transfer loss between capacitors.

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 ex-

ternal 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, R

out ,is

a function of the ON resistance of the internal MOSFET

switches, the oscillator frequency, the capacitance and the

ESR of both C

1 and C2. Since the switching current charging

and discharging C

1 is approximately twice as the output cur-

rent, the effect of the ESR of the pumping capacitor C

1 will

be multiplied by four in the output resistance. The output ca-

pacitor C

2 is charging and discharging at a current approxi-

mately equal to the output current, therefore, this ESR term

only counts once in the output resistance. A good approxi-

mation of R

out is:

where R

SW is the sum of the ON resistance of the internal

MOSFET switches shown in

Figure 2.

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 C

Again, using a low ESR capacitor will result in lower ripple.

Capacitor Selection

The output resistance and ripple voltage are dependent on

the capacitance and ESR values of the external capacitors.

The output voltage drop is the load current times the output

resistance, and the power efficiency is

Where I

Q(V+) is the quiescent power loss of the IC device,

and I

out is the conversion loss associated with the switch

on-resistance, the two external capacitors and their ESRs.

The selection of capacitors is based on the specifications of

the dropout voltage (which equals I

out Rout), the output volt-

age ripple, and the converter efficiency. Low ESR capacitors

(following table) are recommended to maximize efficiency,

reduce the output voltage drop and voltage ripple.

Low ESR Capacitor Manufacturers

Manufacturer

Phone

Capacitor Type

Nichicon Corp.

(708)-843-7500

PL & PF series, through-hole aluminum electrolytic

AVX Corp.

(803)-448-9411

TPS series, surface-mount tantalum

Sprague

(207)-324-4140

593D, 594D, 595D series, surface-mount tantalum

Sanyo

(619)-661-6835

OS-CON series, through-hole aluminum electrolytic

DS100137-26

FIGURE 2. Voltage Inverting Principle

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