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| 型号： | ISL6567IRZ-TS2698 |
| 厂商： | Intersil |
| 文件页数： | 23/25页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 24-QFN |
| 标准包装： | 6,000 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.5MHz |
| 占空比： | 66% |
| 电源电压： | 4.9 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 24-VFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
ISL6567
Similarly, the upper MOSFET begins conducting as soon as it
begins turning on. Assuming the inductor current is in the
positive domain, the upper MOSFET sees approximately the
input voltage applied across its drain and source terminals,
while it turns on and starts conducting the inductor current.
This transition occurs over a time t 2 , and the approximate the
power loss is P UMOS,2 .
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR determines the output ripple voltage and
the initial voltage drop following a high slew-rate transient’s
edge. In most cases, multiple capacitors of small case size
perform better than a single large case capacitor.
Bulk capacitor choices include aluminum electrolytic, OS-Con,
P UMOS , 2 ≈ V IN ? ----------- – ------------ ? ? ---- 2 - ? f S
? I OUT I L , PP ? ? t ?
? N 2 ? ? 2 ?
(EQ. 28)
Tantalum and even ceramic dielectrics. An aluminum
electrolytic capacitor’s ESR value is related to the case size with
lower ESR available in larger case sizes. However, the equivalent
A third component involves the lower MOSFET’s reverse-
recovery charge, Q RR . Since the lower MOSFET’s body diode
conducts the full inductor current before it has fully switched to
the upper MOSFET, the upper MOSFET has to provide the
charge required to turn off the lower MOSFET’s body diode.
This charge is conducted through the upper MOSFET across
VIN, the power dissipated as a result, P UMOS,3 can be
approximated as:
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. Consult the capacitor manufacturer and/or measure
the capacitor’s impedance with frequency to help select a
suitable component.
OUTPUT INDUCTOR SELECTION
P UMOS , 3 = V IN Q rr f S
(EQ. 29)
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
Lastly, the conduction loss part of the upper MOSFET’s power
dissipation, P UMOS,4, can be calculated using Equation 30.
current. In a multi-phase converter, small inductors reduce the
response time with less impact to the total output ripple
current (as compared to single-phase converters).
I PP2
? I OUT ?
P UMOS , 4 = r DS ( ON ) ? ----------- ? d + ----------
2
? N ? 12
(EQ. 30)
1.0
In this case, of course, r DS(ON) is the ON resistance of the
upper MOSFET.
The total power dissipated by the upper MOSFET at full load
can be approximated as the summation of these results. Since
the power equations depend on MOSFET parameters, choosing
the correct MOSFETs can be an iterative process that involves
repetitively solving the loss equations for different MOSFETs
and different switching frequencies until converging upon the
best solution.
0.8
0.6
0.4
0.2
OUTPUT CAPACITOR SELECTION
0
0
0.1
0.2
0.3
0.4
0.5
The output capacitor is selected to meet both the dynamic
load requirements and the voltage ripple requirements. The
load transient a microprocessor impresses is characterized by
high slew rate (di/dt) current demands. In general, multiple
high quality capacitors of different size and dielectric are
paralleled to meet the design constraints.
Should the load be characterized by high slew rates, attention
should be particularly paid to the selection and placement of
DUTY CYCLE (V O /V IN )
FIGURE 27. RIPPLE CURRENT vs DUTY CYCLE
The output inductor of each power channel controls the ripple
current. The control IC is stable for channel ripple current
(peak-to-peak) up to twice the average current. A single
channel’s ripple current is approximated by:
I L , PP = ---------------------------- × -------------
high-frequency decoupling capacitors (MLCCs, typically
multi-layer ceramic capacitors). High frequency capacitors
supply the initially transient current and slow the load
F SW ? L V IN
V IN – V OUT V OUT
(EQ. 31)
rate-of-change seen by the bulk capacitors. The bulk filter
capacitor values are generally determined by the ESR (effective
series resistance) and capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load, or for that reason, to any
decoupling target they are meant for, as physically possible.
The current from multiple channels tend to cancel each other
and reduce the total ripple current. The total output ripple
current can be determined using the curve in Figure 27; it
provides the total ripple current as a function of duty cycle and
number of active channels, normalized to the parameter
K NORM at zero duty cycle.
L ? F SW
Attention should be paid as 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
V OUT
K NORM = --------------------
(EQ. 32)
load on specific decoupling requirements.
23
where L is the channel inductor value.
FN9243.4
August 9, 2011
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