LM2798MMX-2.0 datasheet(10/14 Pages) NSC

No de pièce	LM2798MMX-2.0

Description	120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring

Download	14 Pages

Scroll/Zoom	100%

Fabricant	NSC [National Semiconductor (TI)]

Site Internet	http://www.national.com

Logo

10 / 14 page

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

IN = 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,

R

OUT, because the magnitude of the droop increases lin-

early with load current.

Ideal Charge Pump: V

OUT =GxVIN

Real Charge Pump: V

OUT =(GxVIN)-(IOUT xROUT)

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

(G

B)

Gain Hop

Setting

3.0V - 3.3V

2

⁄3

1

3.8V - 4.4V

1

⁄2

2

⁄3

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

V

IN

EN

V

OUT

POK State

Internal POK Transistor State

V(POK)

>1.7V

H

>95% of V

OUT-nom

HIGH

OFF

V

OUT

>1.7V

H

≤ 92% OF V

OUT-nom

LOW

ON

0V

>1.7V

L

X

LOW

ON

0V

<1.7V

X

LOW

OFF

0V, (V

OUT off)

www.national.com

10

Numéro de pièce similaire - LM2798MMX-2.0

Fabricant	No de pièce	Fiches technique	Description
National Semiconductor ...	LM2791	226Kb / 11P	Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
Texas Instruments	LM2791	1Mb / 18P	[Old version datasheet] LM2791 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
National Semiconductor ...	LM2791LD-H	226Kb / 11P	Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
National Semiconductor ...	LM2791LD-L	226Kb / 11P	Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
Texas Instruments	LM2791LD-L	1Mb / 18P	[Old version datasheet] LM2791 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control

More results

Description similaire - LM2798MMX-2.0

Fabricant	No de pièce	Fiches technique	Description
National Semiconductor ...	LM2788	372Kb / 12P	120mA High Efficiency Step-Down Switched Capacitor Voltage Converter
ON Semiconductor	NCV894530	88Kb / 8P	High Efficiency Low Voltage Step-Down Converter September, 2015 ??Rev. 2
ON Semiconductor	NCV894630	88Kb / 9P	High Efficiency Low Voltage Step-Down Converter September, 2015 ??Rev. 1
National Semiconductor ...	LM2664	222Kb / 9P	Switched Capacitor Voltage Converter
National Semiconductor ...	LM2767	194Kb / 9P	Switched Capacitor Voltage Converter
TelCom Semiconductor, I...	TC828A	67Kb / 8P	Switched Capacitor Voltage Converter
National Semiconductor ...	LM2766	220Kb / 9P	Switched Capacitor Voltage Converter
Analog Intergrations Co...	AIC7660	317Kb / 11P	Switched-Capacitor Voltage Converter
Linear Technology	LT1054L	294Kb / 16P	Switched-Capacitor Voltage Converter with Regulator
Texas Instruments	156869	653Kb / 9P	[Old version datasheet] Switched Capacitor Voltage Converter

More results

LM2798MMX-2.0 Fiches technique(PDF) 10 Page - National Semiconductor (TI)

LM2798MMX-2.0 Fiches technique(HTML) 10 Page - National Semiconductor (TI)

Numéro de pièce similaire - LM2798MMX-2.0

Description similaire - LM2798MMX-2.0

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Fiches technique Télécharger

Lien URL