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参数资料
| 型号： | LM828MWC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHED CAPACITOR CONVERTER, 56 kHz SWITCHING FREQ-MAX, UUC |
| 封装： | WAFER |
| 文件页数： | 6/10页 |
| 文件大小： | 221K |
| 代理商： | LM828MWC |
Pin Description
Pin
Name
Function
1
OUT
Negative voltage output.
2
V+
Power supply positive input.
3
CAP
Connect this pin to the negative terminal of the charge-pump capacitor.
4
GND
Power supply ground input.
5
CAP+
Connect this pin to the positive terminal of the charge-pump capacitor.
Circuit Description
The LM828 contains four large CMOS switches which are
switched in a sequence to invert the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors.
Figure 2 illustrates the voltage conversion scheme.
When S
1 and S3 are closed, C1 charges to the supply
voltage V+. During this time interval, switches S
2 and S4 are
open. In the second time interval, S
1 and S3 are open; at the
same time, S
2 and S4 are closed, C1 is charging C2. After a
number of cycles, the voltage across C
2 will be pumped to
V+. Since the anode of C
2 is connected to ground, the output
at the cathode of C
2 equals (V+) when there is no load
current. The output voltage drop when a load is added is
determined by the parasitic resistance (R
ds(on) of the MOS-
FET switches and the ESR of the capacitors) and the charge
transfer loss between capacitors.
Application Information
Simple Negative Voltage Converter
The main application of LM828 is to generate a negative
supply voltage. The voltage inverter circuit uses only two
external capacitors as shown in the Basic Application Cir-
cuits. The range of the input supply voltage is 1.8V to 5.5V.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistance. The
voltage source equals (V+). The output resistance, R
out ,is
a function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and the
ESR of both C
1 and C2. Since the switching current charging
and discharging C
1 is approximately twice as the output
current, the effect of the ESR of the pumping capacitor C
1
will be multiplied by four in the output resistance. The output
capacitor C
2 is charging and discharging at a current ap-
proximately equal to the output current, therefore, this ESR
term only counts once in the output resistance. A good
approximation of R
out is:
where R
SW is the sum of the ON resistance of the internal
MOSFET switches shown in
Figure 2.
High capacitance, low ESR capacitors will reduce the output
resistance.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, the capacitance and ESR of the output
capacitor C
2:
Again, using a low ESR capacitor will result in lower ripple.
Capacitor Selection
The output resistance and ripple voltage are dependent on
the capacitance and ESR values of the external capacitors.
The output voltage drop is the load current times the output
resistance, and the power efficiency is
Where I
Q(V+) is the quiescent power loss of the IC device,
and I
L
2R
out is the conversion loss associated with the switch
on-resistance, the two external capacitors and their ESRs.
The selection of capacitors is based on the specifications of
the dropout voltage (which equals I
out Rout), the output volt-
age ripple, and the converter efficiency. Low ESR capacitors
(following table) are recommended to maximize efficiency,
reduce the output voltage drop and voltage ripple.
Low ESR Capacitor Manufacturers
Manufacturer
Phone
Capacitor Type
Nichicon Corp.
(708)-843-7500
PL & PF series, through-hole aluminum electrolytic
AVX Corp.
(803)-448-9411
TPS series, surface-mount tantalum
Sprague
(207)-324-4140
593D, 594D, 595D series, surface-mount tantalum
Sanyo
(619)-661-6835
OS-CON series, through-hole aluminum electrolytic
Murata
(800)-831-9172
Ceramic chip capacitors
DS100137-26
FIGURE 2. Voltage Inverting Principle
LM828
www.national.com
5
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