参数资料
| 型号： | ISL6421ER-T |
| 厂商： | Intersil |
| 文件页数： | 7/18页 |
| 文件大小： | 0K |
| 描述： | IC REG SGL LNB CONTROL 32-QFN |
| 标准包装： | 6,000 |
| 应用： | 转换器，卫星信号接收机顶盒设计 |
| 输入电压： | 8 V ~ 14 V |
| 输出数： | 1 |
| 输出电压： | 13 V ~ 18 V |
| 工作温度： | -20°C ~ 85°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（5x5） |
| 包装： | 带卷 (TR) |
Application Note 1083
Inductor
The ISL6421/ISL6425 operates with a 33μH standard
ESR numbers, and for this reason, to improve efficiency and
output ripple, select a capacitor with higher voltage ratings.
inductor over the entire range of supply voltages and load
currents. Choose an inductor that can handle at least the
peak switch current without saturating, and ensure that the
VENDOR
TABLE 3.
SERIES
PACKAGE
inductor has a low DCR (series resistance) to maximize
Sanyo
OS-CON Electrolytic SMD/Through hole
efficiency. The inductor saturation current must be greater
then the switch peak current,
Panasonic
Nichicon
EEUPC Electrolytic
PL Electrolytic
TH
TH
V SW ( max ) ? I OUT V IN ( min ) ? V IN ( min ) ?
2L ? f SW ?
n ? V IN ( min )
I PEAK = ---------------------------------------------- + ----------------------- ? 1 – ---------------------------- ?
where,
L = Inductance, 33μH
V SW ( max ) ?
(EQ. 1)
Sense Resistor
The current sense resistor provides current loop feedback
and sets the overcurrent limit for static current mode. This
resistor value is calculated based on peak switch current per
Equation 2,
R SC < -----------------------
f SW = PWM switching frequency, 220kHz Typical
n = Efficiency, 92% at maximum load
V SENSE
I PEAK
(EQ. 2)
TABLE 2. RECOMMENDED INDUCTORS
where Vsense is 200mV typical (see datasheet specification
VENDOR
Coilcraft
Falco
PART NUMBER
MSS-1260-333MX
SD1016
ISAT
(A)
2.2
2.8
DCR
(m ? )
75
66
PACKAGE
SMD
SMD
table) and I PEAK is calculated from Equation 1. Make sure
the R SC value is always lower than the V SENSE /I PEAK ratio.
For the typical application conditions (V CC = 12V, I OUT
(max) = 450mA) a 100m ? R SC value is a good choice.
Output Capacitors
The most important parameter for the output capacitors is
effective series resistance (ESR). The output ripple is
directly proportional to output capacitor ESR value.
If V IN < 10.5V the inductor peak current can be close to 2A,
then, it is necessary to decrease the R SC value.
See Table 4 for some suggested SMD resistor part numbers.
TABLE 4.
A 68μF or less aluminum electrolytic output filter capacitor
with ESR lower than 80m ? in parallel with a 470nF ceramic
capacitor is a good choice in most application conditions.
The ceramic capacitor is necessary to reduce the high
frequency switching noise.
VENDOR
Meggitt
SEI Electronics
Panasonic
RL73
RMC1
ERJ
SERIES
A high output capacitance and low ESR will strongly reduce
the output ripple voltage and output switching noise and will
improve efficiency. Use the lowest possible ESR capacitor
for best performance.
The maximum value output capacitor is restricted by
transition time specifications between 13V to 18V. With a
high output capacitor the boost circuit will need higher peak
current from input supply to make transition from 13V to 18V
in a given transition time as set by TCAP value. Figure 14
shows the TCAP capacitor value versus transition time. Use
high TCAP capacitor value for high output capacitors to
allow sufficient time to charge the output capacitors in
maximum load conditions.
The capacitor ’s voltage rating should be at least 35V, but
higher voltage electrolytic capacitors generally have lower
7
Layout Guidelines
Just like all switching power supplies, a proper PC board
layout is very important for a single channel
ISL6421/ISL6425 based power supply implementation.
Protect sensitive analog grounds by using a star ground
configuration. Also, minimize lead lengths to reduce stray
capacitance, trace resistance, and radiated noise. Minimize
ground noise by connecting PGND, the input bypass
capacitor ground lead, and the output filter capacitor ground
lead to a single point. Place bypass capacitors as close as
possible to the BYP pin and PGND and the DC/DC output
capacitor as close as possible to VSW.
Place the TCAP capacitor very close to the IC pins and use
the shortest possible ground return path.
Thermal Design
During normal operation, the ISL6421/ISL6425 dissipates
some power. The power dissipation of the output linear
regulator dominates the total power dissipated in the
AN1083.1
May 6, 2005
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