lm2798mm-2.0 National Semiconductor Corporation, lm2798mm-2.0 Datasheet - Page 10

• Manufacturer Part Number: lm2798mm-2.0
• Description: 120ma High Efficiency Step-down Switched Capacitor Voltage Converter With Voltage Monitoring
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
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
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
early with load current.
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.
OUT
Real Charge Pump: V
>
>
>
<
V
1.7V
1.7V
1.7V
1.7V
, because the magnitude of the droop increases lin-
IN
TABLE 3. Typical POK functionality, with 1MΩ pull-up resistor connected between POK and V
Ideal Charge Pump: V
EN
H
H
X
L
OUT
≤ 92% OF V
>
95% of V
= (G x V
V
OUT
OUT
X
X
OUT-nom
OUT-nom
IN
= G x V
(Continued)
) - (I
IN
OUT
= 2.9V, 3.8V),
IN
x R
OUT
POK State
HIGH
)
LOW
LOW
LOW
10
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.
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
(1MΩ (typ.)), or other pull-up device. With a pull-up in place,
V(POK) will be HIGH when V
the nominal output voltage (V
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.
Internal POK Transistor State
TABLE 2. LM2798-1.8 Gain Hopping Regions
Input Voltage
3.0V - 3.3V
3.8V - 4.4V
OFF
OFF
ON
ON
Base Gain
(G
OUT
OUT-nom
2
1
⁄
⁄
B
3
2
)
is at or above 95% (typ) of
= 1.5V, 1.8V, or 2.0V,
0V, (V
Gain Hop
Setting
V(POK)
V
OUT
0V
0V
2
OUT
1
OUT
⁄
3
off)