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參數(shù)資料
| 型號： | MAX1875 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | Dual 180∑ Out-of-Phase PWM Step- Down Controllers with POR |
| 中文描述： | 雙路、180°異相、PWM降壓型控制器，帶有POR |
| 文件頁數(shù)： | 10/21頁 |
| 文件大?。?/td> | 493K |
| 代理商： | MAX1875 |
M
High-Side Gate-Drive Supply (BST_)
Gate-drive voltages for the high-side N-channel switch-
es are generated by the flying-capacitor boost circuits
(Figure 3). A boost capacitor (connected from BST_ to
LX_) provides power to the high-side MOSFET driver.
On startup, the synchronous rectifier (low-side MOSFET)
forces LX_ to ground and charges the boost capacitor to
5V. On the second half-cycle, after the low-side MOSFET
turns off, the high-side MOSFET is turned on by closing
an internal switch between BST_ and DH_. This provides
the necessary gate-to-source voltage to turn on the high-
side switch, an action that boosts the 5V gate-drive
signal above V
IN
. The current required to drive the high-
side MOSFET gates (f
SWITCH
Q
G
) is ultimately drawn
from V
L
.
MOSFET Gate Drivers (DH_, DL_)
The DH and DL drivers are optimized for driving moder-
ate-size N-channel high-side and larger low-side power
MOSFETs. This is consistent with the low-duty factor
seen with large V
IN
- V
OUT
differential. The DL_ low-side
drive waveform is always the complement of the DH_
high-side drive waveform (with controlled dead time to
prevent cross-conduction or
“
shoot-through
”
). An adap-
tive dead-time circuit monitors the DL_ output and pre-
vents the high-side FET from turning on until DL_ is fully
off. There must be a low-resistance, low-inductance
path from the DL_ driver to the MOSFET gate in order
for the adaptive dead-time circuit to work properly.
Otherwise, the sense circuitry in the MAX1875/MAX1876
interprets the MOSFET gate as
“
off
”
while there is actu-
ally charge still left on the gate. Use very short, wide
traces (50mils to 100mils wide if the MOSFET is 1in from
the device). The dead time at the DH-off edge is deter-
mined by a fixed 30ns internal delay.
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal low-side Schottky
catch diode with a low-resistance MOSFET switch.
Additionally, the MAX1875/MAX1876 uses the synchro-
nous rectifier to ensure proper startup of the boost gate-
driver circuit and to provide the current-limit signal.
The internal pulldown transistor that drives DL_ low is
robust, with a 0.5
(typ) on-resistance. This low on-
resistance helps prevent DL_ from being pulled up dur-
ing the fast rise-time of the LX_ node, due to capacitive
coupling from the drain to the gate of the low-side syn-
chronous-rectifier MOSFET. However, for high-current
applications, some combinations of high- and low-side
FETs can cause excessive gate-drain coupling, leading
to poor efficiency, EMI, and shoot-through currents.
This can be remedied by adding a resistor (typically
less than 5
) in series with BST_, which increases the
turn-on time of the high-side FET without degrading the
turn-off time (Figure 3).
Current-Limit Circuit (ILIM_)
The current-limit circuit employs a
“
valley
”
current-sens-
ing algorithm that uses the on-resistance of the low-side
MOSFET as a current-sensing element. If the current-
sense signal is above the current-limit threshold, the
MAX1875/MAX1876 does not initiate a new cycle
(Figure 4). Since valley current sensing is employed, the
actual peak current is greater than the current-limit
threshold by an amount equal to the inductor ripple cur-
rent. Therefore, the exact current-limit characteristic and
maximum load capability are a function of the low-side
MOSFET
’
s on-resistance, current-limit threshold, induc-
tor value, and input voltage. The reward for this uncer-
tainty is robust, lossless overcurrent sensing that does
not require costly sense resistors.
The adjustable current limit accommodates MOSFETs
with a wide range of on-resistance characteristics (see
the
Design Procedure
section). The current-limit thresh-
old is adjusted with an external resistor at ILIM_ (Figure
1). The adjustment range is from 50mV to 300mV, cor-
responding to resistor values of 100k
to 600k
. In
adjustable mode, the current-limit threshold across the
low-side MOSFET is precisely 1/10th the voltage seen
at ILIM_. However, the current-limit threshold defaults
to 100mV when ILIM is tied to V
L
. The logic threshold
for switchover to this 100mV default value is approxi-
mately V
L
- 0.5V.
Adjustable foldback current limit reduces power dissi-
pation during short-circuit conditions (see the
Design
Procedure
section).
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the cur-
rent-sense signals seen by LX_ and PGND. The IC
must be mounted close to the low-side MOSFET with
short, direct traces making a Kelvin sense connection
so that trace resistance does not add to the intended
sense resistance of the low-side MOSFET.
Undervoltage Lockout and Startup
If V
L
drops below 4.5V, the MAX1875/MAX1876 assumes
that the supply and reference voltages are too low to
make valid decisions and activates the undervoltage lock-
out (UVLO) circuitry which forces DL and DH low to inhibit
switching.
RST
is also forced low during UVLO. After V
L
rises above 4.5V, the controller powers up the outputs.
Enable (EN), Soft-Start, and Soft-Stop
Pull EN high to enable or low to shutdown both regula-
tors. During shutdown the supply current drops to 1mA
(max), LX enters a high-impedance state (DH_ con-
nected to LX_, and DL_ connected to PGND), and
COMP_ is discharged to GND through a 17
resistor.
V
L
and REF remain active in shutdown. For
“
always-on
”
operation, connect EN to V
L
.
Dual 180° Out-of-Phase PWM Step-
Down Controllers with POR
10
______________________________________________________________________________________
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MAX1875AEEG | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| MAX1875AEEG+ | 功能描述:電壓模式 PWM 控制器 Dual 180 Out PWM Step-Down RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| MAX1875AEEG+T | 功能描述:電壓模式 PWM 控制器 Dual 180 Out PWM Step-Down RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| MAX1875AEEG-T | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| MAX1875EEG | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
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