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| 型号： | LTC3733CUHF-1#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/32页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR BUCK 3PH AMD CPU 38-QFN |
| 标准包装： | 2,500 |
| 应用： | 控制器，AMD |
| 输入电压： | 4 V ~ 36 V |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 1.55 V |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 38-WFQFN 裸露焊盘 |
| 供应商设备封装： | 38-QFN（5x7） |
| 包装： | 带卷 (TR) |
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LTC3733/LTC3733-1
APPLICATIO S I FOR ATIO
The basic application circuit is shown in Figure 1 on the
first page of this data sheet. External component selection
is driven by the load requirement, and normally begins
with the selection of an inductance value based upon the
desired operating frequency, inductor current and output
voltage ripple requirements. Once the inductors and
operating frequency have been chosen, the current sens-
ing resistors can be calculated. Next, the power MOSFETs
and Schottky diodes are selected. Finally, C IN and C OUT
are selected according to the required voltage ripple
requirements. The circuit shown in Figure 1 can be
configured for operation up to a MOSFET supply voltage
of 28V (limited by the external MOSFETs).
Operating Frequency
The IC uses a constant frequency architecture with the
frequency determined by an internal capacitor. This ca-
pacitor is charged by a fixed current plus an additional
current which is proportional to the voltage applied to the
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge and transition losses. In addition to
this basic tradeoff, the effect of inductor value on ripple
current and low current operation must also be consid-
ered. The PolyPhase approach reduces both input and
output ripple currents while optimizing individual output
stages to run at a lower fundamental frequency, enhancing
efficiency.
The inductor value has a direct effect on ripple current. The
inductor ripple current ? I L per individual section, N,
decreases with higher inductance or frequency and in-
creases with higher V IN or V OUT :
? 1 ?
V OUT ? V OUT ?
fL ?
V IN ?
PLLFLTR pin. Refer to the Phase-Locked Loop and Fre-
quency Synchronization and Setup sections for additional
information.
? I L =
?
A graph for the voltage applied to the PLLFLTR pin versus
frequency is given in Figure 3. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 530kHz.
550
450
350
250
where f is the individual output stage operating frequency.
In a PolyPhase converter, the net ripple current seen by the
output capacitor is much smaller than the individual
inductor ripple currents due to the ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
Figure 4 shows the net ripple current seen by the output
capacitors for the different phase configurations. The
output ripple current is plotted for a fixed output voltage as
the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations. As shown in
Figure 4, the zero output ripple current is obtained when:
150
0
0.5 1.0 1.5 2.0
PLLFLTR PIN VOLTAGE (V)
2.5
3733 F03
V OUT
V IN
=
k
N
where k = 1 , 2 , ..., N – 1
Figure 3. Operating Frequency vs V PLLFLTR
3733f
13
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| LTC3733CUHF-1#TR | IC CTRLR BUCK 3PH AMD CPU 38-QFN |
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