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| 型号： | ISL8112IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 22/27页 |
| 文件大小： | 0K |
| 描述： | IC REG DL BCK/LINEAR SYNC 32-QFN |
| 标准包装： | 6,000 |
| 拓扑： | 降压（降压）同步（1），线性（LDO）（1） |
| 功能： | 任何功能 |
| 输出数： | 2 |
| 频率 - 开关： | 可调式 |
| 电压/电流 - 输出 1： | 控制器 |
| 电压/电流 - 输出 2： | 0.7 V ~ 4.5 V，200mA |
| 带 LED 驱动器： | 无 |
| 带监控器： | 无 |
| 带序列发生器： | 无 |
| 电源电压： | 4.5 V ~ 25 V |
| 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（5x5） |
| 包装： | 带卷 (TR) |
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ISL8112
R3 = R4 ? ? ------------------- – 1 ?
? V ?
Adjustable-Output Feedback (Dual-Mode FB)
Connect FB1 to GND to enable the fixed 5V or tie FB1 to
VCC to set the fixed 1.5V output. Connect a resistive
voltage-divider at FB1 between output and GND to adjust
the respective output voltage between 0.7V and 5.5V
(Figure 27). Choose R2 to be approximately 10k and solve
for R1 using Equation 5.
VR
OUT2
where:
? VR = 2V nominal (if tied to VREF1)
or
? VR = 3.3V nominal (if tied to VREF2)
(EQ. 6)
? V OUT1 ?
? V FB1
UGATE_
UGATE1
R1 = R2 ? ? ------------------- – 1 ?
?
where V FB1 = 0.7V nominal.
UG1
ISL8112 ISL6236
ISL88732
ISL88733
ISL88734
LGATE1
LGATE_ LG1
OUT1 VSEN1
VOUT_
FB_
FB1 FB1
VIN
Q3
Q4
R1
R2
(EQ. 5)
OUT1
Design Procedure
Establish the input voltage range and maximum load current
before choosing an inductor and its associated ripple-current
ratio (LIR). The following four factors dictate the rest of the
design:
1. Input Voltage Range. The maximum value ( V IN (MAX))
must accommodate the maximum AC adapter voltage.
The minimum value ( V IN (MIN)) must account for the
lowest input voltage after drops due to connectors, fuses
and battery selector switches. Lower input voltages result
in better efficiency.
2. Maximum Load Current. The peak load current
(ILOAD(MAX)) determines the instantaneous component
stress and filtering requirements and thus drives output
capacitor selection, inductor saturation rating and the
design of the current-limit circuit. The continuous load
current (ILOAD) determines the thermal stress and drives
the selection of input capacitors, MOSFETs and other
critical heat-contributing components.
3. Switching Frequency. This choice determines the basic
trade-off between size and efficiency. The optimal
frequency is largely a function of maximum input voltage
FIGURE 27. SETTING V OUT1 WITH A RESISTOR DIVIDER
Likewise, connect OUT2REF to VCC to enable the fixed
3.3V or tie OUT2REF to VREF2 to set the fixed 1.05V
output. Set OUT2REF from 0 to 2.50V for SMPS2 tracking
mode (Figure 28).
VIN
and MOSFET switching losses.
4. Inductor Ripple Current Ratio (LIR). LIR is the ratio of the
peak-peak ripple current to the average inductor current.
Size and efficiency trade-offs must be considered when
setting the inductor ripple current ratio. Low inductor
values cause large ripple currents, resulting in the
smallest size, but poor efficiency and high output noise.
The minimum practical inductor value is one that causes
the circuit to operate at critical conduction (where the
UGATE2
UGATE_
LGATE2
LGATE_
VOUT_
OUT2
FB_
REFIN2
UG2
ISL8112 ISL6236
ISL88732
ISL88733
ISL88734
LG2
VSEN2
OUT2REF
R4
Q1
Q2
R3
VR
OUT2
inductor current just touches zero with every cycle at
maximum load). Inductor values lower than this grant no
further size-reduction benefit.
The ISL8112 pulse-skipping algorithm (MODE = GND)
initiates skip mode at the critical conduction point, so the
inductor's operating point also determines the load
current at which PWM/PFM switch over occurs. The
optimum point is usually found between 20% and 50%
ripple current.
Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as follows:
V OUT_ ( V IN + V OUT_ )
V IN ? f ? LIR ? I LOAD ( MAX )
FIGURE 28. SETTING V OUT2 WITH A VOLTAGE DIVIDER FOR
TRACKING
22
L = ---------------------------------------------------------------------
(EQ. 7)
FN6396.1
August 10, 2010
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