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參數(shù)資料
| 型號： | MAX762 |
| 廠商： | Maxim Integrated Products, Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters |
| 中文描述： | 12V/15V或可調(diào)、高效率、低IQ、升壓型DC-DC轉(zhuǎn)換器 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大小： | 122K |
| 代理商： | MAX762 |
used in a bootstrapped circuit (Figure 5), undervoltage
lockout prevents start-up at low input voltages; but
once started, operation can continue down to a lower
voltage that depends on the load.
There is no undervoltage lockout when the internal feed-
back resistors are used (Figure 2), and special circuitry
guarantees start-up at 2.0V. The start-up circuitry fixes
the duty cycle at 50% until V+ is driven to 2.5V, above
which the normal control system takes over.
Shutdown Mode
The MAX761/MAX762 enter shutdown mode when
SHDN is high. In this mode, the internal biasing circuitry
is turned off (including the reference) and V
OUT
equals
V+ minus a diode drop (due to the DC path from the
input to the output). In shutdown mode, the supply cur-
rent drops to less than 5μA. SHDN is a TTL/CMOS logic
level input. Connect SHDN to GND for normal operation.
LBO is high impedance during shutdown.
Modes of Operation
When delivering high output currents, the MAX761/
MAX762 operate in CCM. In this mode, current always
flows in the inductor, and the control circuit adjusts the
switch’s duty cycle on a cycle-by-cycle basis to maintain
regulation without exceeding the switch-current capabili-
ty. This provides excellent load-transient response and
high efficiency.
In discontinuous-conduction mode (DCM), current
through the inductor starts at zero, rises to a peak value,
then ramps down to zero on each cycle. Although effi-
ciency is still excellent, the switch waveforms contain
ringing (the inductor's self-resonant frequency). This
ringing is normal and poses no operational problems.
Low-Battery Detec tor
The MAX761/MAX762 provide a low-battery comparator
that compares the voltage on LBI to the 1.5V reference
voltage. When the LBI voltage is below V
REF
, LBO (an
open-drain output) goes low. The low-battery compara-
tor’s 20mV of hysteresis adds noise immunity, prevent-
ing repeated triggering of LBO. Use a resistor-divider
network between V+, LBI, and GND to set the desired
trip voltage V
TRIP
(Figure 3). When SHDN is high, LBI is
ignored and LBO is high impedance. The value of
resistor R3 should be no larger than 500k
to ensure
the LBI leakage current does not cause inaccuracies in
V
TRIP
.
__________________Design Proc edure
S etting the Output V oltage
The MAX761/MAX762’s output voltage can be adjusted
from 5V to 16.5V using external resistors R1 and R2
configured as shown in Figures 3 and 5. For adjustable-
output operation, select feedback resistor R1 in the
10k
to 250k
range. Higher R1 values within this
range give lowest supply current and best light-load
efficiency. R2 is given by:
R2 = (R1)(V
OUT
- 1)
V
REF
where V
REF
= 1.5V.
Note: Tie FB to GND for fixed-output operation
(bootstrapped mode only).
M
12V /15V or Adjustable, High-Effic ienc y,
Low I
Q
, Step-Up DC-DC Converters
_______________________________________________________________________________________
9
Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)
Figure 4b. Light/Medium Load Current Waveform (500mA/div)
1A
500mA
1A
500mA
0A
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