參數(shù)資料
型號(hào): MAX1708
廠商: Maxim Integrated Products, Inc.
元件分類: DC/DC變換器
英文描述: High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter
中文描述: 高頻、大功率、低噪聲、升壓型DC-DC轉(zhuǎn)換器
文件頁(yè)數(shù): 10/13頁(yè)
文件大?。?/td> 332K
代理商: MAX1708
M
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
10
______________________________________________________________________________________
troller functional diagram. The MAX1708 enters syn-
chronized current-mode PWM when a clock signal
(350kHz < f
CLK
< 1MHz) is applied to CLK. For wire-
less or noise-sensitive applications, this ensures that
switching harmonics are predictable and kept outside
the IF frequency band(s). High-frequency operation
permits low-magnitude output ripple voltage and mini-
mum inductor and filter capacitor size. Switching loss-
es will increase at higher frequencies (see
MAX1708 IC
Power Dissipation
).
Setting the Output Voltage
The MAX1708 features Dual Mode
operation. When
FB is connected to ground, the MAX1708 generates a
fixed output voltage of either +3.3V or +5V, depending
on the logic applied to the 3.3/5 input (Figure 1). The
output can be configured for other voltages, using two
external resistors as shown in Figure 4. To set the out-
put voltage externally, choose an R3 value that is large
enough to minimize load at the output but small enough
to minimize errors due to leakage and the time constant
to FB. A value of R4
50k
is required
where V
FB
= 1.24V.
Soft-Start/Current Limit Adjustment
(SS/LIM)
The soft-start pin allows the soft-start time to be adjust-
ed by connecting a capacitor from SS/LIM to GND.
Select capacitor C3 (see Figure 1):
t
SS
= 4ms + [110
C3 (in μF)]
where t
SS
is the time (in milliseconds) it takes output to
reach its final value.
To improve efficiency or reduce inductor size at
reduced load currents, the current limit can be reduced
from its nominal value (see
Electrical Characteristics
).
A resistor (R1 in Figure 1) between SS/LIM and ground
reduces the current limit as follows:
where I
1
is the desired current limit in amperes and R1
312k
. I
LIM
= 5A, if R1 is omitted.
R
k
I
1
I
LIM
1
312
=
×
R
R
V
V
OUT
FB
3
4
1
=
ONA
0
0
1
1
ONB
0
1
0
1
MAX1708
On
Off
On
On
Figure 2. Simplified Functional Diagram
Table 2. Selecting Switching Frequency
Table 1. On/Off Logic Control
CLK
0
1
MODE
Not allowed
PWM
External clock
(350kHz
1000kHz)
Synchronized PWM
N
PWM
CONTROLLER
STARTUP
OSCILLATOR
OSC
UNDERVOLTAGE LOCKOUT
FB
EN
D
EN
Q
600kHz
OSCILLATOR
EN
REFERENCE
ON
RDY
DUAL MODE
FB
LX
PGND
1.26V
2.15V
OUT
ONA
ONB
REF
CLK
FB
3.3/5
GND
IC POWER
MAX1708
OUT
Dual Mode is a trademark of Maxim Integrated Products.
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