Datasheet LM828MWC, LM828MDC Datasheet (NSC)

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 corre­sponding negative voltage of −1.8V to −5.5V. The LM828
uses two low cost capacitors to provide up to 25 mA of
output current.

The LM828 operates at 12 kHz switching frequency to re­duce output resistance and voltageripple.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

n Inverts Input Supply Voltage
n SOT-23-5 Package
n 20Ω Typical Output Impedance
n 97% Typical Conversion Efficiency at 5 mA

Applications

n Cellular Phones
n Pagers
n PDAs
n Operational Amplifier Power Supplies
n Interface Power Supplies
n Handheld Instruments

Basic Application Circuits

Voltage Inverter

DS100137-1

+5V to −10V Converter

DS100137-2

July 2000

LM828 Switched Capacitor Voltage Converter

© 2001 National Semiconductor Corporation DS100137 www.national.com

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

A

=

25˚C)(Note 3)

240 mW

T

JMax

(Note 3) 150˚C

θ

JA

(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 TJ= 25˚C, and limits in boldface type apply over the full operating temperature range. Un­less otherwise specified: V+ = 5V, C

1=C2

= 10 µF. (Note 4)

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage R

L

=10kΩ 1.8 5.5 V

I

Q

Supply Current No Load 40 75 µA

115

R

OUT

Output Resistance (Note 5) IL= 5 mA 20 65 Ω

f

OSC

Oscillator Frequency (Note 6) Internal 12 24 56 kHz

f

SW

Switching Frequency (Note 6) Measured at CAP+ 6 12 28 kHz

P

EFF

Power Efficiency IL= 5 mA 97 %

V

OEFF

Voltage Conversion Efficiency No Load 95 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
temperatures 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−TA

)/θJA, where T

JMax

is the maximum junction temperature, TAis the

ambient temperature, and θ

JA

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

Note 4: In the test circuit, capacitors C

1

and C2are 10 µF, 0.3Ω 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

=2fSW.

Note 7: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.

LM828

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Test Circuit

Typical Performance Characteristics

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

DS100137-3

*

C1and C2are 10 µF capacitors.

FIGURE 1. LM828 Test Circuit

Supply Current vs
Supply Voltage

DS100137-29

Supply Current vs
Temperature

DS100137-30

Output Source Resistance
vs Supply Voltage

DS100137-31

Output Source Resistance
vs Temperature

DS100137-32

LM828

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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 ’S’ identifies the part as a switchedcapacitorconverter. The next two numbers are the device number.Largerquantityreelsare 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

LM828

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Pin Description

Pin Name Function

1 OUT Negative voltage output.
2 V+ Power supply positive input.
3 CAP− Connect this pin to the negative terminal of the charge-pump capacitor.
4 GND Power supply ground input.
5 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 2

illustrates the voltage conversion scheme.

When S

1

and S3are closed, C1charges to the supply

voltage V+. During this time interval, switches S

2

and S4are

open. In the second time interval, S

1

and S3are open; at the

same time, S

2

and S4are closed, C1is 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
current. The output voltage drop when a load is added is
determined 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
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, 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

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

1

will be multiplied by four in the output resistance. The output
capacitor C

2

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 R

out

is:

where RSWis 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

2

:

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 IQ(V+) is the quiescent power loss of the IC device,
and I

L

2

R

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

outRout

), the output volt­age 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
Murata (800)-831-9172 Ceramic chip capacitors

DS100137-26

FIGURE 2. Voltage Inverting Principle

LM828

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Application Information (Continued)
Low ESR Capacitor Manufacturers (Continued)

Manufacturer Phone Capacitor Type

Taiyo Yuden (800)-348-2496 Ceramic chip capacitors
Tokin (408)-432-8020 Ceramic chip capacitors

Other Applications

Paralleling Devices

Any number of LM828s can be paralleled to reduce the
output resistance. Each device must have its own pumping
capacitor C

1

, while only one output capacitor C

out

is needed

as shown in Figure 3. The composite output resistance is:

Cascading Devices

Cascading the LM828s is an easy way to produce a greater
negative voltage (e.g. A two-stage cascade circuit is shown
in Figure 4).

If n is the integer representing the number of devices cas­caded, the unloaded output voltage V

out

is (-nVin). The ef­fective output resistance is equal to the weighted sum of
each individual device:

R

out

=nR

out_1

+ n/2 R

out_2

+...+R

out_n

This can be seen by first assuming that each device is 100
percent efficient. Since the output voltage is different on
each device the output current is as well. Each cascaded
device sees less current at the output than the previous so
the R

OUT

voltage drop is lower in each device added. Note
that, the number of n is practically limited since the increas­ing of n significantly reduces the efficiency, and increases
the output resistance and output voltage ripple.

Combined Doubler and Inverter

DS100137-9

FIGURE 3. Lowering Output Resistance by Paralleling Devices

DS100137-10

FIGURE 4. Increasing Output Voltage by Cascading Devices

LM828

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Other Applications (Continued)

In Figure 5, the LM828 is used to provide a positive voltage
doubler and a negative voltage converter. Note that the total
current drawn from the two outputs should not exceed 40
mA.

Regulating V

OUT

It is possible to regulate the negative output of the LM828 by
use of a low dropout regulator (such as the LP2980). The
whole converter is depicted in Figure 6. This converter can
give a regulated output from −1.8V to −5.5V by choosing the
proper resistor ratio:

V

out=Vref

(1+R1/R2)

where, V

ref

= 1.23V

Note that the following conditions must be satisfied simulta­neously for worst case design:

V

in_min

>

V

out_min+Vdrop_max

(LP2980)

+I

out_maxxRout_max

(LM828)

V

in_max

<

V

out_max+Vdrop_min

(LP2980)

+I

out_minxRout_min

(LM828)

DS100137-11

FIGURE 5. Combined Voltage Doubler and Inverter

DS100137-12

FIGURE 6. Combining LM828 with LP2980 to Make a Negative Adjustable Regulator

LM828

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Physical Dimensions inches (millimeters) unless otherwise noted

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5-Lead Small Outline Package (M5)

NS Package Number MF05A

For Order Numbers, refer to the table in the ’Ordering Information’ section of this document.

LM828 Switched Capacitor Voltage Converter

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.