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參數(shù)資料
| 型號： | LM2788MMX-1.8 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter |
| 中文描述： | SWITCHED CAPACITOR REGULATOR, 500 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | MSOP-8 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 372K |
| 代理商： | LM2788MMX-1.8 |
Operation Description
OVERVIEW
The LM2788 is a switched capacitor converter that produces
a regulated low-voltage output. The core of the part is the
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 LM2788 is broken up into the following
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 LM2788 achieves tightly regulated output voltages with
pulse-frequency modulated (PFM) regulation. PFM simply
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 refresh the output
capacitor by activating the charge pump, and the supply
current it consumes.
FRACTIONAL MULTI-GAIN CHARGE PUMP
The core of the LM2788 is a two-phase charge pump con-
trolled by an internally generated non-overlapping clock. The
charge pump operates by using the external flying capaci-
tors, 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,
where the fully charged flying capacitors are connected to
the output so that the charge they hold can supply the load
and recharge the output capacitor.
Input, output, and intermediary connections of the flying
capacitors are made with internal MOS switches. The
LM2788 utilizes two flying capacitors and a versatile switch
network to achieve several fractional voltage gains:
1
2
,
2
3
,
and 1. With this gain-switching ability, it is as if the LM2788
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 LM2788 efficiency throughout the
operating ranges of the part. Charge-pump efficiency is de-
rived in the following two ideal equations (supply current and
other losses are neglected for simplicity):
I
IN
= G x I
OUT
E = (V
OUT
x I
OUT
) ÷ (V
IN
x I
IN
) = V
OUT
÷ (G X V
IN
)
In the equations, G represents the charge pump gain. Effi-
ciency is optimal as GxV
approaches V
. Optimal effi-
ciency 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 LM2788 consists of the gains 1/2,
2
3
, and
1. An internal input voltage range detector, along with the
nominal output voltage of the given LM2788 option, deter-
mines what is to be referred to as the ’base gain’ of the part,
G
B
. The base gain is the default gain configuration of the part
at a given V
. Table 1 lists G
of the LM2788-1.8 over the
input voltage range. (For the remainder of this discussion,
the 1.8V option of the LM2788 will be used as an example.
The other voltage options operate under the same principles
as the 1.8V version, the gain-transitions merely occur at
different voltage levels.)
TABLE 1. LM2788-1.8 Base Gain (G
B
) vs. V
IN
Input Voltage
2.6V - 2.9V
2.9V - 3.8V
3.8V - 5.5V
Base Gain (G
B
)
1
2
3
1
2
Table 1
shows the efficiency of the LM2788-1.8 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
, showing the
ideal efficiency of a charge pumps with gains of 1/2, 2/3, and
1. These curves were generated using the ideal efficiency
equation presented above.
The 10mA-load efficiency curve in Figure 1 closely re-
sembles the ideal Efficiency-vs.-Input- Voltage curves that
correspond to each of the base-gain regions. The same
20044422
FIGURE 1. Efficiency of LM2788-1.8 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
( E
IDEAL(G=1)
, E
IDEAL(G=2/3)
, E
IDEAL(G=1/2)
)
L
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8
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