參數(shù)資料
型號(hào): ISL653031EVAL1
廠商: Intersil Corporation
英文描述: Dual 5V Synchronous Buck Pulse-Width Modulator (PWM) Controller for DDRAM Memory VDDQ and VTT Termination
中文描述: 雙同步降壓5V的脈寬調(diào)制(PWM)控制器,用于數(shù)據(jù)存儲(chǔ)器內(nèi)存VDDQ和VTT終端
文件頁數(shù): 12/17頁
文件大小: 540K
代理商: ISL653031EVAL1
12
FN9052.2
November 15, 2004
.
Compensation Break Frequency Equations
Figure 9 shows an asymptotic plot of the DC-DC converter’s
gain vs frequency. The actual modulator gain has a high gain
peak due to the high Q factor of the output filter and is not
shown in Figure 9. Using the above guidelines should give 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
with the capabilities of the error
amplifier. The closed loop gain is constructed on the graph of
Figure 9 by adding the modulator gain (in dB) to the
compensation gain (in dB). This is equivalent to multiplying
the modulator transfer function to the compensation transfer
function and plotting the gain.
The compensation gain uses external impedance networks
Z
FB
and Z
IN
to provide a stable, high bandwidth (BW) overall
loop. A stable control loop has a gain crossing with
-20dB/decade slope and a phase margin greater than 45
degrees. Include worst case component variations when
determining phase margin.
Component Selection Guidelines
Output Capacitor Selection
An output capacitor is required to filter the output and supply
the load transient current. The filtering requirements are a
function of the switching frequency and the ripple current.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
These requirements are generally met with a mix of
capacitors and careful layout.
Modern digital ICs can produce high transient load slew
rates. High-frequency capacitors initially supply the transient
and slow the current load rate seen by the bulk capacitors.
The bulk filter capacitor values are generally determined by
the ESR (effective series resistance) and voltage rating
requirements rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient. An
aluminum electrolytic capacitor’s ESR value is related to the
case size with lower ESR available in larger case sizes.
However, the equivalent series inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacitor to high slew-rate transient loading.
Unfortunately, ESL is not a specified parameter. Work with
your capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In
most cases, multiple electrolytic capacitors of small case size
perform better than a single large case capacitor.
FIGURE 8. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN
V
OUT
REFERENCE
L
O
C
O
ESR
V
IN
DV
OSC
ERROR
AMP
PWM
DRIVER
(PARASITIC)
Z
FB
+
-
REFERENCE
R
1
R
3
R
2
C
3
C
1
C
2
COMP
V
OUT
FB
Z
FB
ISL6530
Z
IN
COMPARATOR
DRIVER
DETAILED COMPENSATION COMPONENTS
PHASE
V
E/A
+
-
+
Z
IN
OSC
F
Z2
+
2
x R
1
R
3
x C
3
------------------------------------------------------
=
F
P1
2
π
x R
2
x
2
1
C
2
+
---------------------
--------------------------------------------------------
=
F
P2
3
3
-----------------------------------
=
F
Z1
2
2
----------------------------------
=
FIGURE 9. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
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
F
LC
F
ESR
COMPENSATION
GAIN
G
FREQUENCY (Hz)
MODULATOR
GAIN
LOOP GAIN
20
V
OSC
---------------
log
20
-------
log
ISL6530
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