LM2797/LM2798 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring

LM2797/LM2798

120mA High Efficiency Step-Down Switched Capacitor

Voltage Converter with Voltage Monitoring

General Description

The LM2797/98 switched capacitor step-down DC/DC con-

verters efficiently produce a 120mA regulated low-voltage

rail from a 2.6V to 5.5V input. Fixed output voltage options of

1.5V, 1.8V, and 2.0V are available. The LM2797/98 uses

multiple fractional gain configurations to maximize conver-

sion efficiency over the entire input voltage and output cur-

rent ranges. Also contributing to high overall efficiency is the

extremely low supply current of the LM2797/98: 35µA oper-

ating unloaded and 0.1µA in shutdown.

Features of the LM2797/98 include input voltage and output

voltage monitoring. Pin BATOK provides battery monitoring

by indicating when the input voltage is above 2.85V (typ.).

Pin POK verifies that the output voltage is not more than 5%

(typ.) below the nominal output voltage of the part.

The optimal external component requirements of the

LM2797/98 solution minimize size and cost, making the part

ideal for Li-Ion and other battery powered designs. Two 1µF

flying capacitors and two 10µF bypass capacitors are all that

is required, and no inductors are needed.

The LM2797/98 also features short-circuit protection over-

temperature protection, and soft-start circuitry to prevent

excessive inrush currents. The LM2798 has a 400µs turn-on

time. The turn-on time of the LM2797 is 100µs.

Features

Output voltage options:

2.0V

5%, 1.8V

5%, and 1.5V

120mA output current capability

Multi-Gain and Gain Hopping for Highest Possible

Efficiency - up to 90% Efficient

2.6V to 5.5V input range

Input and Output Voltage Monitoring (BATOK and POK)

Low operating supply current: 35µA

Shutdown supply current: 0.1µA

Thermal and short circuit protection

LM2798 turn-on time: 400µs

LM2797 turn-on time: 100µs

Available in an 10-Pin MSOP Package

Applications

Cellular Phones

Pagers

H/PC and P/PC Devices

Portable Electronic Equipment

Handheld Instrumentation

Typical Application Circuit

20044501

April 2003

LM2797/LM2798

120mA

High

Efficiency

Step-Down

Switched

Capacitor

oltage

Converter

with

oltage

Monitoring

DS200445

www.national.com

Connection Diagram

LM2797/98

Mini SO-10 (MSOP-10) Package

NS Package #: MUB10A

20044502

Top View

Pin Description

Pin

Name

Description

OUT

Regulated Output Voltage

C1-

First Flying Capacitor: Negative Terminal

C1+

First Flying Capicitor: Positive terminal

Input Voltage. Recommended V

Range: 2.6V to 5.5V

POK

Power-OK Indicator: Output voltage sense. Open-drain NFET output. With an

external pull-up resistor tied to POK, V(POK) will be high when V

OUT

regulating correctly. When V

OUT

falls out of regulation, the internal open-drain

FET pulls the POK voltage low.

BATOK

Battery-OK Indicator: Input voltage sense. Open-drain NFET output. With an

external pull-up resistor tied to BATOK, V(BATOK) will be high when V

2.85V (typ). LM2797/98 pulls V(BATOK) low when V

2.65V (typ.) , and/or

when the part is in shutdown [V(EN) = 0].

Enable Logic Input. High voltage = ON, Low voltage = SHUTDOWN

C2+

Second Flying Capacitor: Positive Terminal

GND

Ground Connection

C2-

Second Flying Capacitor: Negative Terminal

Ordering Information

Nominal

Output

Voltage

OUT(NOM)

Turn-on

Time

Order Number

Package Marking

Supplied As:

1.80V

100µs

LM2797MM-1.8

S80B

1000 units on Tape-and Reel

LM2797MMX-1.8

3500 units on Tape-and-Reel

1.50V

400µs

LM2798MM-1.5

S56B

1000 units on Tape-and Reel

LM2798MMX-1.5

3500 units on Tape-and-Reel

1.80V

400µs

LM2798MM-1.8

S57B

1000 units on Tape-and Reel

LM2798MMX-1.8

3500 units on Tape-and-Reel

2.00V

400µs

LM2798MM-2.0

S58B

1000 units on Tape-and Reel

LM2798MMX-2.0

3500 units on Tape-and-Reel

LM2797/LM2798

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Absolute Maximum Ratings

(Notes 1,

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

, EN, POK, BATOK pins: Voltage

to Ground (Note 3)

−0.3V to 5.6V

Junction Temperature (T

J-MAX-ABS

)

150˚C

Continuous Power Dissipation

(Note 4)

Internally Limited

OUT

Short-Circuit to GND Duration

(Note 4)

Unlimited

Storage Temperature Range

−65˚C to 150˚C

Lead Temperature

(Soldering, 5 Sec.)

260˚C

ESD Rating (Note 5)

Human-body model:

Machine model

2 kV

200V

Operating Ratings

(Notes 1, 2)

Input Voltage Range

2.6V to 5.5V

Recommended Output Current

Range

0mA to 120mA

Junction Temperature Range

-40˚C to 125˚C

Ambient Temperature Range

(Note 6)

-40˚C to 85˚C

Thermal Information

Thermal Resistance, MSOP-8

220˚C/W

Resistance, MSOP-8 Package

(

) (Note 7)

Electrical Characteristics

(Notes 2, 8)

Limits in standard typeface and typical values apply for T

= 25

C. Limits in boldface type apply over the operating junction

temperature range. Unless otherwise specified: 2.6

≤ V

≤ 5.5V, V(EN) = V

, C

= C

= 1µF, C

= C

OUT

= 10µF. (Note 9)

Symbol

Parameter

Conditions

Min

Typ

Max

Units

LM2797-1.8, LM2798-1.8, LM2798-2.0

OUT

Output Voltage Tolerance

2.8V

≤ V

≤ 4.2V

0mA

≤ I

OUT

≤ 120mA

-5

% of

OUT(nom)

(Note 10)

4.2V

≤ 5.5V

0mA

≤ I

OUT

≤ 120mA

-6

LM2798-1.5

OUT

Output Voltage Tolerance

2.8V

≤ V

≤ 4.2V

0mA

≤ I

OUT

≤ 120mA

-6

% of

OUT(nom)

(Note 10)

4.2V

≤ 5.5V

0mA

≤ I

OUT

≤ 120mA

-6

All Output Voltage Options

Operating Supply Current

OUT

= 0mA

µA

Shutdown Supply Current

V(EN) = 0V

0.1

µA

Output Voltage Ripple

LM2798-1.8: V

= 3.6V, I

OUT

= 120mA

p-p

PEAK

Peak Efficiency

LM2798-1.8: V

= 3.0V, I

OUT

= 60mA

AVG

Average Efficiency over

Li-Ion Input Voltage Range

(Note 11)

LM2798-1.5: 3.0

≤ V

≤ 4.2V, I

OUT

= 60mA

LM2798-1.8: 3.0

≤ V

≤ 4.2V, I

OUT

= 60mA

LM2798-2.0: 3.0

≤ V

≤ 4.2V, I

OUT

= 60mA

Turn-On Time

LM2798, V

=2.6V, I

OUT

=100mA, (Note 12)

400

µs

LM2797, V

=2.6V, I

OUT

=100mA, (Note 12)

100

Switching Frequency

500

kHz

Short-Circuit Current

= 3.6, V

OUT

= 0V

Enable Pin (EN) Characteristics

EN pin Logic-High Input

0.9

EN pin Logic-Low Input

0.4

EN pin input current

= 0V

= 5.5V

LM2797/LM2798

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Electrical Characteristics

(Notes 2, 8)

(Continued)

Limits in standard typeface and typical values apply for T

= 25

C. Limits in boldface type apply over the operating junction

temperature range. Unless otherwise specified: 2.6

≤ V

≤ 5.5V, V(EN) = V

, C

= C

= 1µF, C

= C

OUT

= 10µF. (Note 9)

Symbol

Parameter

Conditions

Min

Typ

Max

Units

POK Characteristics

T-POK

Threshold of output voltage

for POK transition

POK transition L to H

% of

OUT-NOM

(Note 10)

POK transition H to L

Hysterisis

POK-H

POK-high leakage current

V(POK) = 3.6V

µA

POK-L

POL-low pull-down voltage

I(POK) = -100µA

200

300

BATOK Characteristics

T-BATOK

Input voltage threshold for

BATOK transition

BATOK transition L to H

2.85

3.0

BATOK transition H to L

2.4

2.65

Hysterisis

0.20

BATOK-H

BATOK-high leakage

current

V(BATOK) = 3.6V

µA

BATOK-L

BATOK-low pull-down

voltage

I(BATOK) = - 100µA

200

300

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of

the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the

Electrical Characteristics tables.

Note 2: All voltages are with respect to the potential at the GND pin.

Note 3: Voltage on the EN pin must not be brought above V

+ 0.3V.

Note 4: Thermal shutdown circuitry protects the device from permanent damage.

Note 5: The human-body model is a 100 pF capacitor discharged through a 1.5k

Ω resistor into each pin. The machine model is a 200pF capacitor discharged

directly into each pin.

Note 6: Maximum ambient temperature (T

A-MAX

) is dependent on the maximum operating junction temperature (T

J-MAX-OP

= 125

C), the maximum power

dissipation of the device in the application (P

D-MAX

), and the junction-to ambient thermal resistance of the part/package in the application (

), as given by the

following equation: T

A-MAX

= T

J-MAX-OP

- (

x P

D-MAX

). The ambient temperature operating rating is provided merely for convenience. This part may be operated

outside the listed T

rating so long as the junction temperature of the device does not exceed the maximum operating rating of 125

Note 7: Junction-to-ambient thermal resistance is highly dependent on application conditions and PC board layout. In applications where high maximum power

dissipation exists, special care must be paid to thermal dissipation issues. For more information on these topics, please refer to the Power Dissipation section of

this datasheet.

Note 8: All room temperature limits are 100% tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed by correlation

using standard Statistical Quality Control methods (SQC). All limits are used to calculate Average Outgoing Quality Level (AOQL). Typical numbers are not

guaranteed, but do represent the most likely norm.

Note 9: C

, C

OUT

, C

, and C

: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics

Note 10: V

OUT (NOM)

is the nominal output voltage of the part. An example: V

OUT-NOM

of LM2798MM-1.8 is 1.8V.

Note 11: Efficiency is measured versus V

, with V

being swept in small increments from 3.0V to 4.2V. The average is calculated from these measurement results.

Weighting to account for battery voltage discharge characteristics (V

BAT

vs. Time) is not done in computing the average.

Note 12: Turn-on time is measured from when the EN signal is pulled high until the output voltage crosses 90% of its final value. Resistive load used for startup

measurement, with value chosen to give I

OUT

= 100mA when the output voltage is fully established.

LM2797/LM2798

www.national.com

Operation Description

OVERVIEW

The LM2797/98 are switched capacitor converters that pro-

duce a regulated low-voltage output. The core of the parts is

a highly efficient charge pump that utilizes multiple fractional

gains and pulse-frequency modulated (PFM) switching to

minimize power losses over wide input voltage and output

current ranges. A description of the principal operational

characteristics of the LM2797/98 is broken up into the fol-

lowing sections: PFM Regulation, Fractional Multi-Gain

Charge Pump, and Gain Selection for Optimal Efficiency.

Each of these sections refers to the block diagram presented

on the previous page.

PFM REGULATION

The LM2797/98 achieves tightly regulated output voltages

with pulse-frequency modulated (PFM) regulation. PFM sim-

ply means the part only pumps when it needs to. When the

output voltage is above the target regulation voltage, the part

idles and consumes minimal supply-current. In this state, the

load current is supplied solely by the charge stored on the

output capacitor. As this capacitor discharges and the output

voltage falls below the target regulation voltage, the charge

pump activates. Charge/current is delivered to the output

(supplying the load and boosting the voltage on the output

capacitor).

The primary benefit of PFM regulation is when output cur-

rents are light and the part is predominantly in the low-

supply-current idle state. Net supply current is minimal be-

cause the part only occasionally needs to recharge the

output capacitor by activating the charge pump.

FRACTIONAL MULTI-GAIN CHARGE PUMP

The core of the LM2797/98 is a two-phase charge pump

controlled by an internally generated non-overlapping clock.

The charge pump operates by using the external flying ca-

pacitors, C1 and C2, to transfer charge from the input to the

output. During the charge phase, which doubles as the PFM

"idle state", the flying capacitors are charged by the input

supply. The charge pump will be in this state until the output

voltage drops below the target regulation voltage, triggering

the charge pump to activate so that it can deliver charge to

the output. Charge transfer is achieved in the pump phase.

In this phase, the fully charged flying capacitors are con-

nected to the output so that the charge they hold can supply

the load current and recharge the output capacitor.

Input, output, and intermediary connections of the flying

capacitors are made with internal MOS switches. The

LM2797/98 utilizes two flying capacitors and a versatile

switch network to achieve several fractional voltage gains:

⁄

, and 1. With this gain-switching ability, it is as if the

LM2797/98 is three-charge-pumps-in-one. The "active"

charge pump at any given time is the one that will yield the

highest efficiency given the input and output conditions

present.

GAIN SELECTION AND GAIN HOPPING FOR OPTIMAL

EFFICIENCY

The ability to switch gains based on input and output condi-

tions results in optimal efficiency throughout the operating

ranges of the LM2797/98. Charge-pump efficiency is derived

in the following two ideal equations (supply current and other

losses are neglected for simplicity):

= G x I

OUT

E = (V

OUT

x I

OUT

) ÷ (V

x I

) = V

OUT

÷ (G X V

)

In the equations, G represents the charge pump gain. Effi-

ciency is at its highest as GxV

approaches V

OUT

. Optimal

efficiency is achieved when gain is able to adjust depending

on input and output voltage conditions. Due to the nature of

charge pumps, G cannot adjust continuously, which would

be ideal from an efficiency standpoint. But G can be a set of

simple quantized ratios, allowing for a good degree of effi-

ciency optimization.

The gain set of the LM2797/98 consists of the gains 1/2, 2/3,

and 1. An internal input voltage range detector, along with

the nominal output voltage of a given LM2797/98 option,

determines what is to be referred to as the "base gain" of the

part, G

. The base gain is the default gain configuration of

the part over a set V

range. Table 1 lists G

of the LM2798-

1.8 over the input voltage range. For the remainder of this

discussion, the 1.8V option of the LM2798 will be used as an

example. The other voltage options of the LM2798 operate

under the same principles as LM2798-1.8, the gain transi-

tions merely occur at different input voltages. Since the only

difference between the LM2797 and the LM2798 is start-up

time, the modes of operation of the LM2798-1.8 discussed

here are identical to those of the LM2797-1.8.

TABLE 1. LM2798-1.8 Base Gain (G

) vs. V

Input Voltage

Base Gain (G

)

2.6V - 2.9V

2.9V - 3.8V

⁄

3.8V - 5.5V

⁄

Figure 1 shows the efficiency of the LM2798-1.8 versus input

voltage, with output currents of 10mA and 120mA. The base

gain regions (G

) are separated and labeled. There is also a

set of ideal efficiency gradients, E

IDEAL(G=xx)

, showing the

ideal efficiency of a charge pumps with gains of 1/2, 2/3, and

1. These gradients have been generated using the ideal

efficiency equation presented above.

20044522

FIGURE 1. Efficiency of LM2798-1.8 with 10mA and

120mA output currents. Base-gain (G

) regions are

separated and labeled. Ideal efficiency curves of

charge pumps with G =1/2, 2/3, and 1 are included,

and are labelled:

IDEAL(G=1)

, E

IDEAL(G=2/3)

, E

IDEAL(G=1/2)

LM2797/LM2798

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Operation Description

(Continued)

The 10mA load curve in Figure 1 gives a clear picture of how

base-gain affects overall converter efficiency. The "ideal ef-

ficiency gradients" in the figure show the efficiency of ideal

switched capacitor converters with gains of 1, 2/3, and 1/2,

respectively. The 10mA-load efficiency curve closely follows

the ideal efficiency gradients in each of the respective base-

gain regions. At the base-gain transitions (V

= 2.9V, 3.8V),

there are sharp transitions in the 10mA curve because the

LM2797/98 switches base-gains. With a 10mA output cur-

rent there is very little gain hopping (described below), and

the gain of the LM2798-1.8 is equal to the base-gain over the

entire operating input voltage range. Internal supply current

has a minimal impact on efficiency with a 10 mA load. Supply

current does have a small effect, and it the reason why the

10mA load curve is slightly below the ideal efficiency gradi-

ents in each of the base-gain regions. But overall, due to the

lack of gain hopping and the minimal impact of supply cur-

rent on converter efficiency, the 10mA load curve very

closely mirrors the ideal efficiency curves in each of the

respecitve base-gain regions.

The 120mA-load curve in Figure 1 illustrates the effect of

gain hopping on converter efficiency. Gain hopping is imple-

mented to overcome output voltage droop that results from

charge-pump non-idealities. In an ideal charge pump, the

output voltage is equal to the product of the gain and the

input voltage. Non-idealities such as finite switch resistance,

capacitor ESR, and other factors result in the output of

practical charge pumps being below the ideal value. This

output droop is typically modeled as an output resistance,

OUT

, because the magnitude of the droop increases lin-

early with load current.

Ideal Charge Pump: V

OUT

= G x V

Real Charge Pump: V

OUT

= (G x V

) - (I

OUT

x R

OUT

)

The LM2797/98 compensates for output voltage droop un-

der high load conditions by gain hopping. When the base-

gain is not sufficient to keep the output voltage in regulation,

the part will temporarily hop up to the next highest gain

setting to provide an intermittent boost in output voltage.

When the output voltage is sufficiently boosted, the gain

configuration reverts back to the base-gain setting. An ex-

ample: if the input voltage of the LM2798-1.8 is 3.2V, the part

is in the 2/3 base-gain region. If the output voltage droops,

the gain configuration will temporarily hop up to a gain of 1.

It will operate with a gain of 1 until the nominal output voltage

is restored, at which time the gain will hop back down to 2/3.

If the load remains high, the part will continue to hop back

and forth between the base-gain and the next highest gain

setting, and the output voltage will remain in regulation. In

contrast to the base-gain decision, which is made based on

the input voltage, the decision to gain hop is made by

monitoring the voltage at the output of the part.

The 120mA-load efficiency curve in Figure 1 illustrates the

effect of gain hopping on efficiency. Comparing the 120mA

load curve to the 10mA load curve, notice that to the right of

the base-gain transitions the efficiency of the 120mA curve

increases gradually. In contrast, the 10mA curve makes a

very sharp transition. The base-gain of both curves is the

same for both loads. The difference comes in gain hopping.

With the 120mA load, the part operates in the base-gain

setting for a certain percentage of time and in the next-

highest gain setting for the remainder. The percentage of

time spent in an elevated gain configuration decreases as

the input voltage rises, as less gain-hopping boost is re-

quired with increased input voltage. When the input voltage

in a given base-gain region is large enough so that no extra

boost from gain hopping is required, the part operates en-

tirely in the base gain region. This can be seen in the figure

where the 120mA-load efficiency curve follows the ideal

efficiency gradients.

TABLE 2. LM2798-1.8 Gain Hopping Regions

Input Voltage

Base Gain

)

Gain Hop

Setting

3.0V - 3.3V

⁄

3.8V - 4.4V

⁄

Gain hopping contributes to the overall high efficiency of the

LM2797/98. Gain hopping only occurs when required to

keep the output voltage in regulation. This allows the

LM2797/98 to operate in the higher efficiency base-gain

setting as much as possible. Gain hopping also allows the

base-gain transitions to be placed at input voltages that are

as low as practically possible. Doing so maximizes the peaks

and minimizes the valleys of the efficiency "saw-tooth"

curves, maximizing total solution efficiency.

POK: OUTPUT VOLTAGE STATUS INDICATOR

The POK pin is an NMOS-open-drain-logic signal that indi-

cates when the output voltage of the LM2797/98 is at or

above 95% (typ) of the target output voltage. To function

properly, the POK pin must be connected to a pull-up resistor

(1M

Ω (typ.)), or other pull-up device. With a pull-up in place,

V(POK) will be HIGH when V

OUT

is at or above 95% (typ) of

the nominal output voltage (V

OUT-nom

= 1.5V, 1.8V, or 2.0V,

depending on voltage option). If the output falls below 92%

(typ.) of the nominal output voltage, V(POK) will be 0V. There

is hysteresis of 3% between the thresholds. The POK func-

tion is disabled and V(POK) is pulled down to 0V when the

LM2797/98 is in shutdown (EN = 0V). Table 3 is a complete

list of the typical POK regions of operation.

TABLE 3. Typical POK functionality, with 1M

Ω pull-up resistor connected between POK and V

OUT

POK State

Internal POK Transistor State

V(POK)

1.7V

95% of V

OUT-nom

HIGH

OFF

OUT

1.7V

≤ 92% OF V

OUT-nom

LOW

1.7V

LOW

1.7V

LOW

OFF

0V, (V

OUT

off)

LM2797/LM2798