参数资料
| 型号： | ISL95874IRUZ-T |
| 厂商： | Intersil |
| 文件页数： | 23/29页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR PWM 1PHASE GPU 16UTQFN |
| 标准包装： | 3,000 |
| 应用： | 控制器，GPU 内核电源 |
| 输入电压： | 3.3 V ~ 25 V |
| 输出数： | 1 |
| 输出电压： | 0.5 V ~ 5 V |
| 工作温度： | -40°C ~ 100°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-UFQFN 裸露焊盘 |
| 供应商设备封装： | 16-UTQFN（2.6x1.8） |
| 包装： | 带卷 (TR) |
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ISL95874, ISL95875, ISL95876
V O (EQ. 37)
V IN
( I MAX ? ( D – D ) ) + ? x ? I MAX ? ------- ?
12 ?
I IN_RMS = ------------------------------------------------------------------------------------------------------
V O ? ( 1 – D )
I P-P = -----------------------------
V O
V IN ? EFF
General Application Design
Guide
This design guide is intended to provide a high-level explanation of
the steps necessary to design a single-phase buck converter. It is
assumed that the reader is familiar with many of the basic skills
and techniques referenced in the following. In addition to this
guide, Intersil provides complete reference designs that include
schematics, bills of materials, and example board layouts.
Selecting the LC Output Filter
The duty cycle of an ideal buck converter is a function of the
input and the output voltage. This relationship is expressed in
Equation 37:
D = --------
The output inductor peak-to-peak ripple current is expressed in
Equation 38:
(EQ. 38)
F SW ? L
A typical step-down DC/DC converter will have an I P-P of 20% to
40% of the maximum DC output load current. The value of I P-P is
selected based upon several criteria such as MOSFET switching
loss, inductor core loss, and the resistive loss of the inductor
winding. The DC copper loss of the inductor can be estimated
using Equation 39:
Selecting the Input Capacitor
The important parameters for the bulk input capacitors are the
voltage rating and the RMS current rating. For reliable operation,
select bulk capacitors with voltage and current ratings above the
maximum input voltage and capable of supplying the RMS current
required by the switching circuit. Their voltage rating should be at
least 1.25x greater than the maximum input voltage, while a
voltage rating of 1.5x is a preferred rating. Figure 20 is a graph of
the input RMS ripple current, normalized relative to output load
current, as a function of duty cycle that is adjusted for converter
efficiency. The ripple current calculation is written as Equation 42:
2 2 2 2 D
? (EQ. 42)
I MAX
Where:
- I MAX is the maximum continuous I LOAD of the converter
- x is a multiplier (0 to 1) corresponding to the inductor
peak-to-peak ripple amplitude expressed as a percentage of
I MAX (0% to 100%)
- D is the duty cycle that is adjusted to take into account the
efficiency of the converter
Duty cycle is written as Equation 43:
D = ----------------------- (EQ. 43)
P COPPER = I LOAD ? DCR
2
(EQ. 39)
In addition to the bulk capacitors, some low ESL ceramic
Where, I LOAD is the converter output DC current.
The copper loss can be significant so attention has to be given to
the DCR of the inductor. Another factor to consider when
choosing the inductor is its saturation characteristics at elevated
temperature. A saturated inductor could cause destruction of
circuit components, as well as nuisance OCP faults.
capacitors are recommended to decouple between the drain of
the high-side MOSFET and the source of the low-side MOSFET.
0.6
0.5
x=0
0.4
A DC/DC buck regulator must have output capacitance C O into,
which ripple current I P-P can flow. Current I P-P develops a
corresponding ripple voltage V P-P across C O, which is the sum of the
voltage drop across the capacitor ESR and of the voltage change
stemming from charge moved in and out of the capacitor. These two
0.3
0.2
0.1
x=1
x = 0.5
voltages are expressed in Equations 40 and 41:
Δ V ESR = I P-P ? E SR
(EQ. 40)
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
DUTY CYCLE
I P-P
Δ V C = ------------------------------
C BOOT ≥ ---------------------
(EQ. 41)
8 ? C O ? F SW
If the output of the converter has to support a load with high
pulsating current, several capacitors will need to be paralleled to
reduce the total ESR until the required V P-P is achieved. The
inductance of the capacitor can significantly impact the output
voltage ripple and cause a brief voltage spike if the load transient
has an extremely high slew rate. Low inductance capacitors should
be considered. A capacitor dissipates heat as a function of RMS
current and frequency. Be sure that I P-P is shared by a sufficient
quantity of paralleled capacitors so that they operate below the
maximum rated RMS current at F SW . Take into account that the
rated value of a capacitor can fade as much as 50% as the DC
voltage across it increases.
23
FIGURE 20. NORMALIZED INPUT RMS CURRENT FOR EFF = 1
Selecting the Bootstrap Capacitor
The integrated driver features an internal bootstrap Schottky
diode. Simply adding an external capacitor across the BOOT and
PHASE pins completes the bootstrap circuit. The bootstrap
capacitor voltage rating is selected to be at least 10V. Although the
theoretical maximum voltage of the capacitor is PVCC-V DIODE
(voltage drop across the boot diode), large excursions below
ground by the phase node requires at least a 10V rating for the
bootstrap capacitor. The bootstrap capacitor can be chosen from
Equation 44:
Q GATE (EQ. 44)
Δ V BOOT
FN7933.1
March 2, 2012
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