参数资料
| 型号： | ISL6237IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 31/35页 |
| 文件大小： | 0K |
| 描述： | IC MAIN PWR CTRLR QUAD 32-QFN |
| 产品培训模块： | Solutions for Industrial Control Applications |
| 标准包装： | 6,000 |
| 应用： | 控制器，笔记本电脑电源系统 |
| 输入电压： | 5.5 V ~ 25 V |
| 输出数： | 4 |
| 输出电压： | 多重 |
| 工作温度： | -40°C ~ 100°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-VFQFN 裸露焊盘 |
| 供应商设备封装： | 32-QFN 裸露焊盘（5x5） |
| 包装： | 带卷 (TR) |
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ISL6237
UGATE_
UGATE2
.
UGATE2
V IN
Q 1
Determining the Current Limit
The minimum current-limit threshold must be great enough
to support the maximum load current when the current limit
is at the minimum tolerance value. The valley of the inductor
ISL6237
ISL88732
ISL88733
ISL88734
OUT2
current occurs at I LOAD(MAX) minus half of the ripple
current; therefore:
I LIMIT ( LOW ) > I LOAD ( MAX ) – [ ( LIR ? 2 ) ? I LOAD ( MAX ) ]
(EQ. 11)
LGATE2 LGATE2
LGATE_
OUT2
VOUT_ OUT2
FB_
REFIN2 REFIN2
R 4
Q 2
R 3
VR
where: I LIMIT(LOW) = minimum current-limit threshold
voltage divided by the r DS(ON) of Q 2 /Q 4 .
Use the worst-case maximum value for r DS(ON) from the
MOSFET Q 2 /Q 4 data sheet and add some margin for the
rise in r DS(ON) with temperature. A good general rule is to
allow 0.2% additional resistance for each °C of temperature
rise.
Examining the 5A circuit example with a maximum
r DS(ON) = 5m Ω at room temperature. At +125°C reveals the
FIGURE 73. SETTING V OUT2 WITH A VOLTAGE DIVIDER FOR
TRACKING
Inductor Selection
following:
I LIMIT ( LOW ) = ( 25mV ) ? ( ( 5m Ω × 1.2 ) > 5A – ( 0.35 ? 2 ) 5A )
(EQ. 12)
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
4.17A > 4.12A
(EQ. 13)
V OUT _ ( V IN + V OUT _ )
V IN ? f ? LIR ? I LOAD ( MAX )
L = ----------------------------------------------------------------- = 8.3 μ H
L = ---------------------------------------------------------------------
Example: I LOAD(MAX) = 5A, V IN = 12V, V OUT2 = 5V,
f = 200kHz, 35% ripple current or LIR = 0.35:
5V ( 12V – 5V )
12V ? 200kHz ? 0.35 ? 5A
(EQ. 7)
(EQ. 8)
4.17A is greater than the valley current of 4.12A, so the
circuit can easily deliver the full-rated 5A using the 30mV
nominal current-limit threshold voltage.
Output Capacitor Selection
The output filter capacitor must have low enough equivalent
series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR to
satisfy stability requirements. The output capacitance must
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice. The core must be large enough
not to saturate at the peak inductor current (IPEAK):
also be high enough to absorb the inductor energy while
transitioning from full-load to no-load conditions without
tripping the overvoltage fault latch. In applications where the
output is subject to large load transients, the output
IPEAK = I LOAD ( MAX ) + [ ( LIR ? 2 ) ? I LOAD ( MAX ) ]
(EQ. 9)
capacitor's size depends on how much ESR is needed to
prevent the output from dipping too low under a load
R SER ≤ ----------------------------------
The inductor ripple current also impacts transient response
performance, especially at low V IN - V OUT _ differences. Low
inductor values allow the inductor current to slew faster,
transient. Ignoring the sag due to finite capacitance:
V DIP
I LOAD ( MAX )
(EQ. 14)
replenishing charge removed from the output filter capacitors
by a sudden load step. The peak amplitude of the output
transient (V SAG ) is also a function of the maximum duty
factor, which can be calculated from the on-time and
where V DIP is the maximum-tolerable transient voltage drop.
In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
( Δ I LOAD ( MAX ) ) ? L ? K ? ------------------- + t OFF ( MIN ) ? ?
? V IN – V OUT ?
2 ? C OUT ? V OUT K ? -------------------------------- ? - t
R ESR ≤ -----------------------------------------------
V P – P
L IR ? I LOAD ( MAX )
minimum off-time:
2 ? ? V OUT _ ? ?
? ? V IN ? ?
V SAG = ----------------------------------------------------------------------------------------------------------------------------
? V IN ? OFF ( MIN )
(EQ. 10)
(EQ. 15)
where V P-P is the peak-to-peak output voltage ripple. The
actual capacitance value required relates to the physical size
needed to achieve low ESR, as well as to the chemistry of
the capacitor technology. Thus, the capacitor is usually
where minimum off-time = 0.35μs (max) and K is from
Table 2.
31
selected by ESR and voltage rating rather than by
capacitance value (this is true of tantalum, OS-CON, and
other electrolytic-type capacitors).
FN6418.4
March 18, 2008
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