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參數(shù)資料
| 型號： | ISL8502 |
| 廠商： | Intersil Corporation |
| 英文描述： | 2.5A Synchronous Buck Regulator with Integrated MOSFETs |
| 中文描述： | 包2.5a同步降壓穩(wěn)壓器，集成的MOSFET |
| 文件頁數(shù)： | 18/19頁 |
| 文件大?。?/td> | 526K |
| 代理商： | ISL8502 |
18
FN6389.0
January 17, 2007
FIGURE 37. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
Layout Considerations
Layout is very important in high frequency switching
converter design. With power devices switching efficiently
between 500kHz and 1.2MHz, the resulting current
transitions from one device to another cause voltage spikes
across the interconnecting impedances and parasitic circuit
elements. These voltage spikes can degrade efficiency,
radiate noise into the circuit, and lead to device overvoltage
stress. Careful component layout and printed circuit board
design minimizes these voltage spikes.
As an example, consider the turn-off transition of the control
MOSFET. Prior to turn-off, the MOSFET is carrying the full
load current. During turn-off, current stops flowing in the
MOSFET and is picked up by the lower MOSFET. Any
parasitic inductance in the switched current path generates a
large voltage spike during the switching interval. Careful
component selection, tight layout of the critical components,
and short, wide traces minimizes the magnitude of voltage
spikes.
There are two sets of critical components in the ISL8502
switching converter. The switching components are the most
critical because they switch large amounts of energy, and
therefore tend to generate large amounts of noise. Next are
the small signal components which connect to sensitive
nodes or supply critical bypass current and signal coupling.
A multi-layer printed circuit board is recommended. Figure
38 shows the connections of the critical components in the
converter. Note that capacitors CIN and COUT could each
represent numerous physical capacitors. Dedicate one solid
layer, usually a middle layer of the PC board, for a ground
plane and make all critical component ground connections
with vias to this layer. Dedicate another solid layer as a
power plane and break this plane into smaller islands of
common voltage levels. Keep the metal runs from the
PHASE terminals to the output inductor short. The power
plane should support the input power and output power
nodes. Use copper filled polygons on the top and bottom
circuit layers for the phase nodes. Use the remaining printed
circuit layers for small signal wiring. The wiring traces from
the GATE pins to the MOSFET gates should be kept short
and wide enough to easily handle the 1A of drive current.
In order to dissipate heat generated by the internal V
TT
LDO, the ground pad, pin 29, should be connected to the
internal ground plane through at least five vias. This allows
the heat to move away from the IC and also ties the pad to
the ground plane through a low impedance path.
The switching components should be placed close to the
ISL8502 first. Minimize the length of the connections
between the input capacitors, CIN, and the power switches
by placing them nearby. Position both the ceramic and bulk
input capacitors as close to the upper MOSFET drain as
possible. Position the output inductor and output capacitors
between the upper and lower MOSFETs and the load. Make
the PGND and the output capacitors as short as possible.
The critical small signal components include any bypass
capacitors, feedback components, and compensation
components. Place the PWM converter compensation
components close to the FB and COMP pins. The feedback
resistors should be located as close as possible to the FB
pin with vias tied straight to the ground plane as required.
100
80
60
40
20
0
-20
-40
-60
F
P1
F
Z2
10M
1M
100K
10K
1K
100
10
OPEN LOOP
ERROR AMP GAIN
F
Z1
F
P2
20LOG
(R
2
/R
1
)
F
LC
F
ESR
COMPENSATION
GAIN
CLOSED LOOP
GAIN
G
FREQUENCY (Hz)
20LOG
(V
IN
/
Δ
V
OSC
)
MODULATOR
GAIN
FIGURE 38. PRINTED CIRCUIT BOARD POWER PLANES AND
ISLANDS
VIN
PVCC
ISL8502
VCC
PHASE
PGND
COMP
FB
GND PAD
R
4
R
3
C
3
R
1
C
1
C
2
R
2
C
OUT1
V
OUT1
C
IN
V
IN
L
1
C
BP2
R
BP
C
BP1
5V
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER
VIA CONNECTION TO GROUND PLANE
KEY
L
ISL8502
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