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参数资料
| 型号： | ISL8107EVAL2Z |
| 厂商： | Intersil |
| 文件页数： | 14/16页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD 2 FOR ISL8107 |
| 标准包装： | 1 |
| 系列： | * |
ISL8107
6. Calculate R 1 such that the placement of F Z2 is at the F LC .
R 1 = -------------------------------------- – R 3
1
2 π × C 3 × F LC
(EQ. 18)
F Z1 F Z2
F P1
MODULATOR GAIN
COMPENSATION GAIN
LOOP GAIN
OPEN LOOP E/A GAIN
R 4 = ------------------------------------- × R 1
7. Calculate R 4 based on target output voltage.
V REF
V OUT – V REF
(EQ. 19)
F P2
20 log ? -------- ?
MAX ? V IN
OSC
It is recommended that a mathematical model be used to
plot the loop response. Check the loop gain against the error
amplifier ’s open-loop gain. Verify phase margin results and
adjust as necessary. Equations 20 and 21 describe the
0
R2
? R1 ?
D
20 log ----------------------------------
V
G CL
G FB
frequency response of the buck converter in continuous
conduction mode (G vd ), feedback compensation (G comp )
and loop response (G LP ):
LOG
F LC
F CE
F 0
G MOD
FREQUENCY
G vd ( f ) = ------------------------------- ? -----------------------------------------------------------------------------------------------------------
V OSC
1 + s ( f ) ? ( ESR + DCR ) ? C + s ( f ) ? L ? C
D MAX ? V IN 1 + s ( f ) ? ESR ? C
2
FIGURE 16. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
target crossover frequencies in the range of 10% to 30% of
1 + s ( f ) ? R 2 ? C 1
s ( f ) ? R 1 ? ( C 1 + C 2 )
1 + s ( f ) ? ( R 1 + R 3 ) ? C 3
( 1 + s ( f ) ? R 3 ? C 3 ) ? ? 1 + s ( f ) ? R 2 ? --------------------- ?
G COMP ( f ) = ---------------------------------------------------- ?
--------------------------------------------------------------------------------------------------------------------
? C 1 ? C 2 ?
? C 1 + C 2 ?
(EQ. 20)
the switching frequency (F SW ).
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to
another can generate voltage transients across the
impedances of the interconnecting bond wires and circuit
G LP ( f ) = G vd ( f ) ? G COMP ( f )
where , s ( f ) = 2 π ? f ? j
traces. These interconnecting impedances should be
minimized by using wide, short printed circuit traces. The
COMPENSATION BREAK FREQUENCY EQUATIONS
critical components should be located as close together as
possible using ground plane construction or single point
F Z1 = -------------------------------
F Z2 = -------------------------------------------------
F P1 = -------------------------------
F P2 = ---------------------------------------------
2 π ? R 2 ? ---------------------
1
2 π ? R 2 ? C 1
1
2 π ? ( R 1 + R 3 ) ? C 3
1
2 π ? R 3 ? C 3
1
C 1 ? C 2
C 1 + C 2
(EQ. 21)
grounding.
A multi-layer printed circuit board is recommended.
Figure 17 shows the critical components of the converter.
Note that capacitors C IN and C OUT could each represent
numerous physical capacitors. Dedicate one solid layer,
(usually a middle layer of the PC board) for a ground plane
Figure 16 shows an asymptotic plot of the DC/DC converter’s
gain vs frequency. The actual Modulator Gain has a high gain
peak dependent on the quality factor (Q) of the output filter,
which is not shown. Using the previous guidelines should yield
a compensation gain similar to the curve plotted. The open loop
error amplifier gain bounds the compensation gain. Check the
compensation gain at F P2 against the capabilities of the error
amplifier. The loop gain, G LP , is constructed on the log-log
graph of Figure 16 by adding the modulator gain, G vd (in
dB), to the feedback compensation gain, G COMP (in dB).
This is equivalent to multiplying the modulator transfer
function and the compensation transfer function and then
plotting the resulting gain.
A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than 45°.
Include worst case component variations when determining
phase margin. The mathematical model presented makes a
number of approximations and is generally not accurate at
frequencies approaching or exceeding half the switching
frequency. When designing compensation networks, select
14
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.
FN6605.0
October 29, 2008
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