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參數(shù)資料
| 型號(hào)： | MAX660 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | CMOS Monolithic Voltage Converter |
| 中文描述： | CMOS單片電壓變換器 |
| 文件頁(yè)數(shù)： | 6/11頁(yè) |
| 文件大?。?/td> | 259K |
| 代理商： | MAX660 |
Application Information
(Continued)
Again, using a low ESR capacitor will result in lower ripple.
POSITIVE VOLTAGE DOUBLER
The MAX660 can operate as a positive voltage doubler (as
shown in the TypicalApplication Circuits). The doubling func-
tion is achieved by reversing some of the connections to the
device. The input voltage is applied to the GND pin with an
allowable voltage from 2.5V to 5.5V. The V+ pin is used as
the output. The LV pin and OUT pin must be connected to
ground. The OSC pin can not be driven by an external clock
in this operation mode. The unloaded output voltage is twice
of the input voltage and is not reduced by the diode D
1
’s for-
ward drop.
The Schottky diode D
is only needed for start-up. The inter-
nal oscillator circuit uses the V+ pin and the LV pin (con-
nected to ground in the voltage doubler circuit) as its power
rails. Voltage across V+ and LV must be larger than 1.5V to
insure the operation of the oscillator. During start-up, D
is
used to charge up the voltage at V+ pin to start the oscillator;
also, it protects the device from turning-on its own parasitic
diode and potentially latching-up. Therefore, the Schottky di-
ode D
should have enough current carrying capability to
charge the output capacitor at start-up, as well as a low for-
ward voltage to prevent the internal parasitic diode from
turning-on. A Schottky diode like 1N5817 can be used for
most applications. If the input voltage ramp is less than
10V/ms, a smaller Schottky diode like MBR0520LT1 can be
used to reduce the circuit size.
SPLIT V+ IN HALF
Another interesting application shown in the Basic Applica-
tion Circuits is using the MAX660 as a precision voltage di-
vider. Since the off-voltage across each switch equals V
IN
/2,
the input voltage can be raised to +11V.
CHANGING OSCILLATOR FREQUENCY
The internal oscillator frequency can be selected using the
Frequency Control (FC) pin. When FC is open, the oscillator
frequency is 10 kHz; when FC is connected to V+, the fre-
quency increases to 80 kHz. A higher oscillator frequency al-
lows smaller capacitors to be used for equivalent output re-
sistance and ripple, but increases the typical supply current
from 0.12 mA to 1 mA.
The oscillator frequency can be lowered by adding an exter-
nal capacitor between OSC and GND. (See Typical Perfor-
mance Characteristics.) Also, in the inverter mode, an exter-
nal clock that swings within 100 mV of V+ and GND can be
used to drive OSC. Any CMOS logic gate is suitable for driv-
ing OSC. LV must be grounded when driving OSC. The
maximum external clock frequency is limited to 150 kHz.
The switching frequency of the converter (also called the
charge pump frequency) is half of the oscillator frequency.
Note:
OSC cannot be driven by an external clock in the
voltage-doubling mode.
TABLE 1. MAX660 Oscillator Frequency Selection
FC
Open
V+
Open
or V+
OSC
Open
Open
External
Capacitor
Oscillator
10 kHz
80 kHz
See Typical
Performance
Characteristics
External Clock
Frequency
N/A
External Clock
(inverter mode only)
CAPACITOR SELECTION
As discussed in the Simple Negative Voltage Converter sec-
tion, the output resistance and ripple voltage are dependent
on the capacitance and ESR values of the external capaci-
tors. 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
L2
R
is the conversion loss associated with the
switch on-resistance, the two external capacitors and their
ESRs.
Since the switching current charging and discharging C
is
approximately twice as the output current, the effect of the
ESR of the pumping capacitor C
is multiplied by four in the
output resistance. The output capacitor C
is charging and
discharging at a current approximately equal to the output
current, therefore, its ESR only counts once in the output re-
sistance. However, the ESR of C
directly affects the output
voltage ripple. Therefore, low ESR capacitors (Table 2) are
recommended for both capacitors to maximize efficiency, re-
duce the output voltage drop and voltage ripple. For conve-
nience, C
1
and C
2
are usually chosen to be the same.
The output resistance varies with the oscillator frequency
and the capacitors. In Figure 4 the output resistance vs. os-
cillator frequency curves are drawn for three different tanta-
lum capacitors. At very low frequency range, capacitance
plays the most important role in determining the output resis-
tance. Once the frequency is increased to some point (such
as 20 kHz for the 150 μF capacitors), the output resistance is
dominated by the ON resistance of the internal switches and
the ESRs of the external capacitors. A low value, smaller
size capacitor usually has a higher ESR compared with a
bigger size capacitor of the same type. For lower ESR, use
ceramic capacitors.
DS100898-3
FIGURE 3. Splitting V
IN
in Half
M
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