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參數(shù)資料
| 型號： | ML4866 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | 3.3V Output DC-DC Step-Down Converter(3.3V輸出的DC-DC步降變換器) |
| 中文描述： | 3.3V輸出的DC - DC降壓轉換器（3.3V的輸出的的DC - DC步降變換器） |
| 文件頁數(shù)： | 6/8頁 |
| 文件大小： | 128K |
| 代理商： | ML4866 |
ML4866
6
REV. 1.0 10/12/2000
FREQUENCY COMPENSATION
Frequency compensation of the ML4866 is required when
the converter is operating in PWM mode. Two simple
methods are provided to ensure the converter is frequency
stable. Both these methods will work only if the inductor
current is selected to be in CCM at the maximum load
current (see Inductor Selection). The first, called dominant
pole compensation, is used when non-varying loads are
expected. This method requires a single capacitor
connected from the error amplifier output (COMP Pin) to
ground.
For loads which change suddenly, the transient response
(or bandwidth) of the circuit must be increased to prevent
the output voltage from going outside of the regulation
band. The method used to accomplish this is called
zero/pole compensation and requires a series resistor
capacitor network from COMP to ground.
To determine which method works best for a given
application, apply the components found from the
zero/pole compensation method to an actual circuit
and examine the output voltage variation. If the voltage
variation is acceptable, connect the simpler, single
capacitor and re-check the output voltage for acceptable
load transient response.
NON-VARYING LOAD CURRENT
For the best possible response to load transients using only
a single capacitor, dominant pole compensation is
implemented with a single capacitor value of:
C
g
f
COMP
m
COMP
=
×
2
(6)
Where f
COMP
is the unity gain crossover point (640Hz),
g
m
= 62.5μmho, and C
COMP
> 15.5nF (choose a standard
18nF or 22nF capacitor). The value of C
COMP
can be
increased but at the risk of increased output voltage
variations with transient loads.
VARYING LOAD CURRENT
To minimize output voltage variations due to rapidly
changing load currents, use the series RC zero
compensation method to find the compensation network
component values that will improve the output voltage
response to load transients.
The unity gain bandwidth of the converter is extended to
15kHz using an RC network determined by:
R
G
g
where G
f
f
COMP
m
O
COMP
>
=
,
(7)
C
R
COMP
COMP
=
×
1
50
π
(8)
Where f
0
= 15kHz, f
COMP
= 640Hz, R
COMP
> 375k
(use
390k
, 5%), and C
COMP
= 16nF (use 15nF).
Either method of compensation for CCM mode with result
in continued stability as the ML4866 changes to DCM
mode at lighter load currents. Figure 3 shows a typical
application circuit for the ML4866.
VOLTAGE
RATING
ESR @
100kHz
CAPACITANCE
SIZE
4.7μF
16V
3216
0.490
10μF
6.3V
3216
0.368
22μF
16V
7343
0.149
33μF
6.3V
6032
0.291
47μF
10V
7343
0.144
100μF
6.3V
7343
0.088
Table 2. ESR Values for Low Cost Tantalum Capacitors
DESIGN CONSIDERATIONS
(Continued)
Figure 3. Typical Application Circuit
1
2
3
4
8
7
6
5
VOUT
COMP
VREF
BURST
GND
VL
SHDN
VIN
ML4866
15nF
100nF
15nF
390k
VIN
3.5V to 6.5V
33μF
100μF
100nF
VOUT
3.3V
100μH
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相關代理商/技術參數(shù) |
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| ML4866CS | 制造商:MICRO_LINEAR 功能描述: |
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