LM828
Switched Capacitor Voltage Converter

General Description

The LM828 CMOS charge-pump voltage converter inverts a
positive voltage in the range of +1.8V to +5.5V to the corre-
sponding negative voltage of -1.8V to -5.5V. The LM828
uses two low cost capacitors to provide up to 25 mA of out-
put current.

The LM828 operates at 12 kHz switching frequency to re-
duce output resistance and voltage ripple. With an operating
current of only 40 µA (operating efficiency greater than 96%
with most loads), the LM828 provides ideal performance for
battery powered systems. The device is in a tiny SOT-23-5
package.

Features

Inverts Input Supply Voltage

SOT-23-5 Package

Typical Output Impedance

97% Typical Conversion Efficiency at 5 mA

Applications

Cellular Phones

Pagers

PDAs

Operational Amplifier Power Supplies

Interface Power Supplies

Handheld Instruments

Basic Application Circuits

Voltage Inverter

DS100137-1

+5V to -10V Converter

DS100137-2

March 1999

LM828

Switched

Capacitor

oltage

Converter

DS100137

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Absolute Maximum Ratings

(Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Supply Voltage (V+ to
GND, or GND to OUT)

5.8V

V+ and OUT Continuous
Output Current

50 mA

Output Short-Circuit
Duration to GND (Note 2)

1 sec.

Continuous Power
Dissipation (T

25°C)(Note 3)

240 mW

JMax

(Note 3)

150°C

(Note 3)

300°C/W

Operating Junction
Temperature Range

-40°C to 85°C

Storage Temperature
Range

-65°C to +150°C

Lead Temp. (Soldering, 10
seconds)

300°C

ESD Rating (Note 7)

2kV

Electrical Characteristics

Limits in standard typeface are for T

= 25°C, and limits in boldface type apply over the full operating temperature range. Un-

less otherwise specified: V+ = 5V, C

= C

= 10 µF. (Note 4)

Symbol

Parameter

Condition

Min

Typ

Max

Units

V+

Supply Voltage

=10k

1.8

5.5

Supply Current

No Load

µA

115

OUT

Output Resistance (Note 5)

= 5 mA

OSC

Oscillator Frequency (Note 6)

Internal

kHz

Switching Frequency (Note 6)

Measured at CAP+

kHz

EFF

Power Efficiency

= 5 mA

OEFF

Voltage Conversion Efficiency

No Load

99.96

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating conditions.

Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to V+ may damage the device and should be avoided. Also, for tem-
peratures above 85°C, OUT must not be shorted to GND or V+, or the device may be damaged.

Note 3: The maximum allowable power dissipation is calculated by using P

DMax

= (T

JMax

- T

, where T

JMax

is the maximum junction temperature, T

is the

ambient temperature, and

is the junction-to-ambient thermal resistance of the package.

Note 4: In the test circuit, capacitors C

and C

are 10 µF, 0.3

maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output

voltage and efficiency.

Note 5: Specified output resistance includes internal switch resistance and capacitor ESR. See the details in the application information.

Note 6: The output switches operate at one half of the oscillator frequency, f

OSC

= 2f

Note 7: The human body model is a 100 pF capacitor discharged through a 1.5 k

resistor into each pin.

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Typical Performance Characteristics

(Circuit of Figure 1, V+ = 5V unless otherwise

specified) (Continued)

Connection Diagram

Ordering Information

Order Number

Package

Number

Package Marking

Supplied as

LM828M5

MA05B

S08A (Note 8)

Tape and Reel (250 units/rail)

LM828M5X

MA05B

S08A (Note 8)

Tape and Reel (3000 units/rail)

Note 8: The first letter

identifies the part as a switched capacitor converter. The next two numbers are the device number. Larger quantity reels are available upon

request.

Output Voltage
vs Load Current

DS100137-33

Efficiency vs
Load Current

DS100137-34

Switching Frequency vs
Supply Voltage

DS100137-35

Switching Frequency vs
Temperature

DS100137-36

5-Lead Small Outline Package (M5)

DS100137-13

Top View With Package Marking

DS100137-14

Actual Size

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Pin Description

Pin

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

and S

are closed, C

charges to the supply volt-

age V+. During this time interval, switches S

and S

are

open. In the second time interval, S

and S

are open; at the

same time, S

and S

are closed, C

is charging C

. After a

number of cycles, the voltage across C

will be pumped to

V+. Since the anode of C

is connected to ground, the output

at the cathode of C

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

and C

. Since the switching current charging

and discharging C

is approximately twice as the output cur-

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

will

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

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

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

(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

), 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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