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參數(shù)資料
| 型號(hào)： | LM2798MMX-1.5 |
| 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 with Voltage Monitoring |
| 中文描述： | 0.12 A SWITCHED CAPACITOR REGULATOR, 500 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | MSOP-10 |
| 文件頁(yè)數(shù)： | 9/14頁(yè) |
| 文件大小： | 388K |
| 代理商： | LM2798MMX-1.5 |
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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:
1
2
,
2
3
, 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):
I
= G x I
OUT
E = (V
OUT
x I
OUT
) ÷ (V
IN
x I
IN
OUT
÷ (G X V
IN
)
In the equations, G represents the charge pump gain. Effi-
ciency is at its highest as GxV
approaches V
. 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
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
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
, 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:
E
IDEAL(G=1)
, E
IDEAL(G=2/3)
, E
IDEAL(G=1/2)
L
www.national.com
9
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|---|---|
| LM2798MMX-2.0 | 功能描述:IC REG BUCK SWITCHED CAP 10-MSOP RoHS:否 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 開(kāi)關(guān)穩(wěn)壓器 系列:- 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:1 系列:- 類型:降壓（降壓） 輸出類型:固定 輸出數(shù):1 輸出電壓:3.3V 輸入電壓:4.5 V ~ 24 V PWM 型:- 頻率 - 開(kāi)關(guān):- 電流 - 輸出:125mA 同步整流器:無(wú) 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:SOT-23-6 包裝:Digi-Reel® 供應(yīng)商設(shè)備封裝:SOT-6 其它名稱:MAX1836EUT33#TG16DKR |
| LM27A | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:【3∑C Accurate, 120∑C-150∑C Factory Preset Thermostat (LM27 in Die Form) |
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| LM27CIM5-1HJ/NOPB | 功能描述:恒溫器 RoHS:否 制造商:Maxim Integrated 功能: 開(kāi)路溫度: 閉合溫度: 準(zhǔn)確性:+/- 1 C 溫度范圍:- 55 C to + 125 C 電流額定值:500 uA 電壓額定值:2.7 V to 5.5 V 產(chǎn)品:Thermostats |
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