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參數(shù)資料
| 型號(hào)： | DS2282 |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類(lèi)： | Digital Transmission Controller |
| 英文描述： | DATACOM, FRAMER, SMA30 |
| 文件頁(yè)數(shù)： | 3/16頁(yè) |
| 文件大?。?/td> | 312K |
| 代理商： | DS2282 |
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connected to the output voltage (-5V, -12V, -15V). In
nonbootstrapped operation, OUT is connected to
ground, and EXT now swings from V+ to ground.
At high input-to-output differentials, it may be neces-
sary to use nonbootstrapped mode to avoid the 21V V+
to VOUT maximum rating. Also, observe the VGS maxi-
mum rating of the external transistor. At intermediate
voltages and currents, the advantages of bootstrapped
vs. nonbootstrapped operation are slight. When input
voltages are less than about 4V, always use the boot-
strapped circuit.
Shutdown and Quiescent Current
The MAX774/MAX775/MAX776 are designed to save
power in battery-powered applications. A TTL/CMOS
logic-level shutdown input (SHDN) has been provided
for the lowest-power applications. When shut down
(SHDN = V+), most internal bias current sources and
the reference are turned off so that less than 5A of
current is drawn.
In normal operation, the quiescent current will be less
than 100A. However, this current is measured by forc-
ing the external switch transistor off. Even with no load,
in an actual application, additional current will be
drawn to supply the feedback resistors’ and the diode’s
and capacitor’s leakage current. Under no-load condi-
tions, you should see a short current pulse at half the
peak current approximately every 100ms (the exact
period depends on actual circuit leakages).
EXT Drive Voltages
EXT swings from OUT to V+ and provides the drive out-
put for an external power MOSFET. When using the on-
chip feedback resistors for the preset output voltages,
the voltage at OUT equals the output voltage. When
using external feedback resistors, OUT may be tied to
GND or some other potential between VOUT and GND.
Always observe the V+ to OUT absolute maximum rat-
ing of 21V. For V+ to output differentials greater than
21V, OUT must be tied to a potential more positive than
the output and, therefore, the output voltage must be
set with an external resistor divider.
In nonbootstrapped operation with low input voltages
(<4V), tie OUT to a negative voltage to fully enhance the
external MOSFET. Accomplish this by creating an inter-
mediate voltage for VOUT with a zener diode (Figure 5).
__________________Design Procedure
Setting the Output Voltage
The MAX774/MAX775/MAX776 are preset for -5V, -12V,
and -15V output voltages, respectively; however, they
may also be adjusted to other values with an external
voltage divider. For the preset output voltage, connect
FB to REF and connect OUT to the output (Figure 3). In
this case, the output voltage is sensed by OUT.
For an adjustable output (Figures 3 and 4), connect an
external resistor divider from the output voltage to FB,
and from FB to REF. In this case, the divided-down out-
put voltage is sensed via the FB pin.
There are three reasons to use the external resistor divider:
1) An output voltage other than a preset value is
desired.
2) The input-to-output differential exceeds 21V.
3) The output voltage (VOUT to GND) exceeds -15V.
See Figures 3 and 4 for adjustable operation. The
impedance of the feedback network should be low
enough that the input bias current of FB is not a factor.
For best efficiency and precision, allow 10A to flow
through the network. Calculate (VREF -VFB) / R1 =
10A. Since VREF = 1.5V and VFB = 0V, R1 becomes
150k
. Then calculate R2 as follows:
R2
VOUT
___ = _______
R1
VREF
(or, VOUT = 10
A)
______
R2
Choosing an Inductor
Practical inductor values range from 10H to 50H.
The maximum inductor value is not particularly critical.
For highest current at high
VOUT to V+ ratios, the
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable, High-Efficiency,
Low IQ Inverting DC-to-DC Controllers
______________________________________________________________________________________
11
Figure 5. Connection Using Zener Diode to Boost Base Drive
R1
MAX774
MAX775
MAX776
OUT
GND
FB
R2
8
2
1
REF
4
0.1
F
0.1
F
VOUT
RZ
6V
≤ VZ + VIN ≤ 10V
VOUT – VZ
> IZ
RZ
IZ = ZENER BREAKDOWN CURRENT
VZ = ZENER BREAKDOWN VOLTAGE
VIN = INPUT SUPPLY VOLTAGE
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