参数资料
| 型号： | ISL6237IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 33/35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
current limit and cause the fault latch to trip. To protect
against this possibility, "overdesign" the circuit to tolerate:
during each switching cycle and V SAG greatly increases
unless additional output capacitance is used.
I LOAD = I LIMIT ( HIGH ) + ( ( LIR ) ? 2 ) ? I LOAD ( MAX )
(EQ. 21)
A reasonable minimum value for h is 1.5, but this can be
adjusted up or down to allow trade-offs between V SAG,
where I LIMIT(HIGH) is the maximum valley current allowed
by the current-limit circuit, including threshold tolerance and
resistance variation.
output capacitance and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as shown in Equation 22:
t OFF ( MIN ) ? h
1 – ? ------------------------------------ ?
Rectifier Selection
Current circulates from ground to the junction of both
MOSFETs and the inductor when the high-side switch is off.
( V OUT _ + V DROP )
V IN ( MIN ) = --------------------------------------------------- + V DROP2 – V DROP1
? K ?
(EQ. 22)
I L ? r DS ( ON )
As a consequence, the polarity of the switching node is
negative with respect to ground. This voltage is
approximately -0.7V (a diode drop) at both transition edges
while both switches are off (dead time). The drop is
when the low-side switch conducts.
The rectifier is a clamp across the synchronous rectifier that
catches the negative inductor swing during the dead time
between turning the high-side MOSFET off and the
synchronous rectifier on. The MOSFETs incorporate a
high-speed silicon body diode as an adequate clamp diode if
efficiency is not of primary importance. Place a Schottky diode
in parallel with the body diode to reduce the forward voltage
drop and prevent the Q 2 /Q 4 MOSFET body diodes from
turning on during the dead time. Typically, the external diode
improves the efficiency by 1% to 2%. Use a Schottky diode
with a DC current rating equal to one-third of the load current.
where V DROP1 and V DROP2 are the parasitic voltage drops
Specifications on page 3 and K is taken from Table 2. The
absolute minimum input voltage is calculated with h = 1.
Operating frequency must be reduced or h must be increased
and output capacitance added to obtain an acceptable V SAG
if calculated V IN(MIN) is greater than the required minimum
input voltage. Calculate V SAG to be sure of adequate
transient response if operation near dropout is anticipated.
Dropout Design Example:
ISL6237: With V OUT2 = 5V, fsw = 400kHz, K = 2.25μs,
t OFF(MIN) = 350ns, V DROP1 = V DROP2 = 100mV, and
h = 1.5, the minimum V IN is:
V IN ( MIN ) = ---------------------------------------------- + 0.1V – 0.1V = 6.65V
1 – ? ------------------------------- ?
V IN ( MIN ) = ----------------------------------------- + 0.1V – 0.1V = 6.04V
1 – ? -------------------------- ?
For example, use an MBR0530 (500mA-rated) type for loads
up to 1.5A, a 1N5817 type for loads up to 3A, or a 1N5821
type for loads up to 10A. The rectifier's rated reverse
breakdown voltage must be at least equal to the maximum
input voltage, preferably with a 20% derating factor.
Applications Information
Dropout Performance
( 5V + 0.1V )
0.35 μ s ? 1.5
? 2.25 μ s ?
Calculating with h = 1 yields:
( 5V + 0.1V )
0.35 μ s ? 1
? 2.25 μ s ?
(EQ. 23)
(EQ. 24)
The output voltage-adjust range for continuous-conduction
operation is restricted by the nonadjustable 350ns (max)
minimum off-time one-shot. Use the slower 5V SMPS for the
higher of the two output voltages for best dropout
performance in adjustable feedback mode. The duty-factor
limit must be calculated using worst-case values for on- and
off-times, when working with low input voltages.
Manufacturing tolerances and internal propagation delays
introduce an error to the TON K-factor. Also, keep in mind that
transient-response performance of buck regulators operated
close to dropout is poor, and bulk output capacitance must
often be added (see Equation 10 on page 31).
The absolute point of dropout occurs when the inductor
current ramps down during the minimum off-time ( Δ I DOWN ) as
much as it ramps up during the on-time ( Δ I UP ). The ratio
h = Δ I UP / Δ I DOWN indicates the ability to slew the inductor
current higher in response to increased load, and must always
be greater than 1. As h approaches 1, the absolute minimum
dropout point, the inductor current is less able to increase
33
Therefore, V IN must be greater than 6.65V. A practical input
voltage with reasonable output capacitance would be 7.5V.
PC Board Layout Guidelines
Careful PC board layout is critical to achieve minimal switching
losses and clean, stable operation. This is especially true when
multiple converters are on the same PC board where one
circuit can affect the other. Refer to the ISL6237 Evaluation Kit
data sheet for a specific layout example.
Mount all of the power components on the top side of the board
with their ground terminals flush against one another, if
possible. Follow these guidelines for good PC board layout:
? Isolate the power components on the top side from the
sensitive analog components on the bottom side with a
ground shield. Use a separate PGND plane under the
OUT1 and OUT2 sides (called PGND1 and PGND2). Avoid
the introduction of AC currents into the PGND1 and PGND2
ground planes. Run the power plane ground currents on the
top side only, if possible.
FN6418.4
March 18, 2008
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