参数资料
| 型号： | SC412AMLTRT |
| 厂商： | Semtech |
| 文件页数： | 11/22页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 16-MLP |
| 标准包装： | 1 |
| PWM 型： | 控制器 |
| 输出数： | 1 |
| 频率 - 最大： | 325kHz |
| 占空比： | 90% |
| 电源电压： | 3 V ~ 25 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： | 16-MLP |
| 包装： | 标准包装 |
| 其它名称： | SC412AMLDKR |
NOT RECOMMENDED FOR NEW DESIGN
POWER MANAGEMENT
Applications Information (continued)
SC412A
Soft-Start
The soft-start is accomplished by ramping the FB com-
parator’s internal reference from zero to 0.75V in 30mV
increments. Each 30mV step typically lasts for eight clock
cycles.
During the soft-start period, the Zero Cross Detector is
active to monitor the voltage across the lower MOSFET
while DL is high. If the inductor current reaches zero, the
FB comparator’s internal ramp reference is immediately
overridden to match the voltage at the FB pin. This soon
causes the FB comparator to trip which forces DL to turn
off and a DH on-time will begin. This prevents the inductor
current from going too negative which would cause droop
in the VOUT start-up waveform. The next 30mV step on the
internal reference ramp occurs from the new point at the
FB pin. Since any of the internal 30mV steps can be over-
ridden by the FB waveform, the start-up time is therefore
dependent upon operating conditions. This override feature
will stop when the FB pin reaches approximately 660mV.
At start-up, during the ? rst 32 switching cycles, the over-
current threshold is reduced by 50%, to reduce overshoot
caused by the ? rst set of switching pulses.
MOSFET Gate Drivers
The DH and DL drivers are optimized for driving moderate
high-side and larger low-side power MOSFETs. An adaptive
dead-time circuit monitors the DL output and prevents the
high-side MOSFET from turning on until DL is fully off, and
conversely, monitors the DH output and prevents the low-
side MOSFET from turning on until DH is fully off. Be sure
there is low resistance and low inductance between the DH
and DL outputs to the gate of each MOSFET.
The SC412A utilizes SmartDrive TM to achieve fast switching
with reduced noise. At the start of the DH on-time when
LX is typically below GND, the DH output drives the high-
side MOSFET through a pull-up resistance of 10 ohms,
which results in a soft reverse-recovery of the low-side
diode. The high-side MOSFET conducts and causes LX to
rise; when LX reaches 1.5volts, the DH drive resistance is
reduced to 2 ohms to provide fast switching and reduce
switching loss.
Design Procedure
Prior to designing a switch mode supply, the input voltage,
load current, switching frequency and inductor ripple cur-
rent must be speci ? ed.
For notebook systems the maximum input voltage (VIN MAX )
is determined by the highest AC adaptor voltage, and the
minimum input voltage (VIN MIN ) is determined by the lowest
battery voltage after accounting for voltage drops due to
connectors, fuses and battery selector switches.
In general, four parameters are needed to de ? ne the
design:
1) Nominal output voltage (VOUT)
2) Static or DC output tolerance
3) Transient response
4) Maximum load current (IOUT)
There are two values of load current to consider: continu-
ous load current and peak load current. Continuous load
current is concerned with thermal stresses which drive
the selection of input capacitors, MOSFETs and commuta-
tion diodes. Peak load current determines instantaneous
component stresses and ? ltering requirements such as
inductor saturation, output capacitors and design of the
current limit circuit.
Design example:
VBAT = 10V min, 20V max
VOUT = 1.15V +/- 4%
Load = 20A maximum
Inductor Selection
Low inductor values result in smaller size, but create high-
er ripple current and are less ef ? cient because of the high
AC current ? owing in the inductor. Higher inductor values
will reduce the ripple current and are more ef ? cient, but
are larger and more costly. The inductor selection is gen-
erally based on the ripple current which is typically set
between 20% to 50% of the maximum load current. Cost,
size, output ripple and ef ? ciency all play a part in the se-
lection process.
? 2006 Semtech Corp.
11
www.semtech.com
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