参数资料
| 型号： | SC419ULTRT |
| 厂商： | Semtech |
| 文件页数： | 20/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 20-MLPQ |
| 标准包装： | 1 |
| 系列： | EcoSpeed®, SmartDrive™ |
| PWM 型： | 控制器 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 电源电压： | 3 V ~ 28 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： | 20-MLPQ |
| 包装： | 标准包装 |
| 其它名称： | SC419ULTRTDKR |
SC419
Applications Information (continued)
Switching Frequency Variations
The switching frequency will vary depending on line and
load conditions. The line variations are a result of fixed
?
Connect PGND and AGND together with a zero
ohm resistor or copper trace. Make the connec-
tion near the AGND and PGND pins of the IC.
?
?
?
?
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propagation delays in the on-time one-shot, as well as
unavoidable delays in the external MOSFET switching. As
V IN increases, these factors make the actual DH on-time
slightly longer than the ideal on-time. The net effect is
that frequency tends to falls slightly with increasing input
voltage.
The switching frequency also varies with load current as a
result of the power losses in the MOSFETs and the induc-
tor. For a conventional PWM constant-frequency con-
verter, as load increases the duty cycle also increases
slightly to compensate for IR and switching losses in the
MOSFETs and inductor. A adaptive on-time converter
must also compensate for the same losses by increasing
the effective duty cycle (more time is spent drawing
energy from V IN as losses increase). The on-time is essen-
tially constant for a given V OUT /V IN combination, to offset
the losses the off-time will tend to reduce slightly as load
increases. The net effect is that switching frequency
increases slightly with increasing load.
PCB Layout Guidelines
As with any switch-mode converter, good PCB layout is
essential to achieving high performance. The following
guidelines will provide an optimum PCB layout.
The device layout can be considered in four parts;
Power Components
Use short, wide connections between the power compo-
nents:
Input capacitors and high-side MOSFETs
High-side and Low-side MOSFETs and inductor
(LX connection). Use wide copper traces to
provide high current carrying capacity and for
heat dissipation.
Inductor and output capacitors
All PGND connections — the input capacitors,
low-side MOSFETs, output capacitors, and the
PGND pin of the SC419.
? An inner layer ground plane is recom-
mended.
? Each component should have a good,
low impedance connection to the ground
plane.
? Place vias to the PGND plane directly near
the component pins.
Use short wide traces for the pin connections
from the SC419 (LX, DH, DL and BST). Do not
route these traces near the sensitive analog
signals (FB, TON, V OUT ).
Low-Noise Analog Circuits
Low-noise analog circuits are sensitive circuits that are
referenced to AGND. Due to their high impedance and
?
?
?
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Grounding for PGND and AGND
Power Components
Low-noise Analog Circuits
Bypass capacitors
sensitivity to noise, it is important that these circuits be
kept away from the switching signals.
? Use a ground plane or solid copper area for
AGND. Place all components connected to
?
?
Grounding
A ground plane layer for PGND is recommended
to minimize the effects of switching noise, resis-
tive losses, and to maximize heat removal from
the power components.
A separate ground plane or island should be used
for AGND and all associated components. The
AGND island should avoid overlapping the switch-
ing signals on other layers (DH/DL/BST/LX).
?
AGND above this area.
? Use short direct traces for the AGND con-
nections to all components.
? Place vias to the AGND plane directly near
the component pins.
Proper routing of the V OUT sense trace is essential
since it feeds into the FB resistor divider. Noise
on the FB waveform will cause instability and
multiple pulsing.
20
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