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參數(shù)資料
| 型號： | ML4902 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | High Current Synchronous Buck Controller For DC/DC conversion(用于DC/DC變換的大電流同步補償控制器（5V到1.8V～3.5V變換）) |
| 中文描述： | 高電流同步降壓控制器，用于直流/直流轉(zhuǎn)換（用于直流/直流變換的大電流同步補償控制器（5V的到1.8?3.5V的變換）） |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 280K |
| 代理商： | ML4902 |
ML4902
10
LAYOUT ISSUES
The two pins of the ML4902 which actually sense the
current limit voltage are I
SENSE
and GND. To facilitate
the required low-level sensing of the voltage between
these pins, there is no connection inside the ML4902
between GND and PWR GND. Because of this, there must
be an external connection between the ML4902 GND and
PWR GND pins. PWR GND must have a low impedance
connection to the ground plane used on the board, as high
instantaneous currents will flow in PWR GND when N
DRV L and N DRV H switch the capacitive loads of the
output MOSFET gates. At the same time, GND must not
see the resulting switching spikes.
If a current sensing resistor is used, the voltage across the
resistor must be Kelvin-sensed. This ensures that the
ML4902 monitors only the voltage across the resistor, and
ignores the voltage drops and inductive transients in the
PCB traces which carry current into and out of this
resistor. The two pins of the ML4902 which must be
Kelvin-connected to the sense resistor are I
SENSE
and
GND. PWR GND should then return to the to the
grounded end of R
SENSE
as well, using a high current
Kelvin connection. This causes any noise across the
resistor to appear primarily as a common-mode signal on
I
SENSE
, GND, and PWR GND. Figure 4 shows a
recommended implementation of these PCB layout
requirements.
When directly monitoring the voltage across the channel
of the synchronous rectifier, the voltage across that
MOSFET should be sensed as closely as possible to its
drain. If a resistor divider is used to reduce the voltage at
the I
SENSE
pin for a given current through (Q3||Q4)’s
channel resistance, then the lower end of the divider
should be returned to the immediate vicinity of its source.
This ensures that the ML4902 monitors only the voltage
across the synchronous rectifier, and not the voltage drops
or inductive transients in the PCB traces which carry
current into and out of it. If a PC board with a dedicated
ground plane is used (recommended), the best return
points for GND and PWR GND are directly into the
ground plane. If the board does not have a dedicated
ground plane, GND must be returned to a point near the
IC which is relatively free from switching transients. Such
a point may need to be empirically determined but will
usually be near the ground connection of the output
capacitor bank.
MISCELLANEOUS POINTS
I
SENSE
is the input to a medium-speed, high-sensitivity
comparator (roughly comparable to an LM339-type
comparator in terms of speed of response). Because of the
leading-edge blanking on this comparator, it has a
substantial ability to reject switching noise. Still, proper
circuit function requires that the comparator not see
significant noise at the time during which the synchronous
rectifier MOSFET is on.
The compensation components R4 and C13 are high-
impedance nodes connected to the output of the voltage
loop error amplifier. These components should be kept in
close proximity to the ML4902. C13 should be returned to
GND, not to PWR GND or the ground plane of the PC
board.
Keep the V
REF
bypass capacitor C8 close to the ML4902.
Ensure that its ground connection is to GND, not to PWR
GND.
The 12V V
DD
input is the supply from which the internal
circuitry of the ML4902 operates. V
DD
also provides the
gate drive for N DRV H and N DRV L. The V
DD
bypass
capacitors C10 and C20 should be returned to PWR GND
or to the PC board ground plane. They should not be
returned to GND due to high transient currents which
could interfere with the current sensing function.
V
CC
is the input to the 5V undervoltage lockout
comparator circuitry. The 5V UVLO function makes the
start-up of the ML4902 independent of power sequencing.
It also provides additonal overcurrent protection in case
V
CC
should go below acceptable levels (current drawn
from the bulk 5V supply will rise as the actual voltage of
that supply decreases). To reject switching noise on the
5V input, an RC filter should be used between the 5V
source and V
CC
. Typical values for this filter are R2 =
1k
W
, and C11 = 220nf.
Optional capacitor C22 may be needed in some layouts
to filter out “glitches” which could occur on the PWR
GOOD signal. In conjunction with the resistive pullup for
the PWR GOOD line, its value should yield an RC
product of approximately 5μs.
In order to reduce circuit size, complexity, and cost, an
all N-channel power MOSFET output stage is employed.
The gate drive voltage for both the sourcing and the
rectifying MOSFETs is derived from the 12V input bus.
This delivers at least 10V of V
GS
enhancement to the
rectifier MOSFET(s). The power sourcing MOSFET(s),
however, have a worst-case V
GS
enhancement of about
6V, and must therefore be logic-level parts.
If a given design uses power MOSFETs in an 8 pin SOIC
package style, keep in mind that the thermal dissipation
capability of these parts is largely dictated by the copper
area available to their drains. A good layout will
maximize this area.
DESIGN CONSIDERATIONS
(Continued)
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ML4902CT | 制造商:MICRO-LINEAR 制造商全稱:MICRO-LINEAR 功能描述:High Current Synchronous Buck Controller |
| ML4902CT UP WAF | 制造商:Fairchild Semiconductor Corporation 功能描述: |
| ML4902CTX | 制造商:Rochester Electronics LLC 功能描述:- Bulk 制造商:Fairchild Semiconductor Corporation 功能描述: |
| ML4903 | 制造商:MICRO-LINEAR 制造商全稱:MICRO-LINEAR 功能描述:High Current Synchronous Buck Controller |
| ML4903CT | 制造商:Rochester Electronics LLC 功能描述:- Tape and Reel 制造商:Fairchild Semiconductor Corporation 功能描述: |
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