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参数资料
| 型号： | SC4503WLTRT |
| 厂商： | Semtech |
| 文件页数： | 11/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BOOST ADJ 1.4A 8MLPD |
| 标准包装： | 1 |
| 类型： | 升压（升压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调至 27V |
| 输入电压： | 2.5 V ~ 20 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 1.3MHz |
| 电流 - 输出： | 1.4A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-WFDFN 裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 8-MLPD（2x2） |
| 其它名称： | SC4503WLDKR |
SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Switch on duty ratio in DCM is given by,
When the switch is turned on, the output capacitor supplies
=
?
(11)
the load current I OUT (Figure 4). The output ripple voltage
due to charging and discharging of the output capacitor
is therefore:
Higher input current ripples and lower output current are
the drawbacks of DCM operation.
?
=
(13)
Input Capacitor
For most applications, a 10-22 μ F ceramic capacitor is suf-
The input current in a boost converter is the inductor cur- ? cient for output ? ltering. It is worth noting that the output
rent, which is continuous with low RMS current ripples. A ripple voltage due to discharging of a 10 μ F ceramic capaci-
2.2-4.7 μ F ceramic input capacitor is adequate for most tor (13) is higher than that due to its ESR.
applications.
Rectifying Diode
Output Capacitor
For high ef ? ciency, Schottky barrier diodes should be used
Both ceramic and low ESR tantalum capacitors can be as rectifying diodes for the SC4503. These diodes should
used as output ? ltering capacitors. Multi-layer ceramic have an average forward current rating at least equal to the
capacitors, due to their extremely low ESR (<5m Ω ), are output current and a reverse blocking voltage of at least
the best choice. Use ceramic capacitors with stable a few volts higher than the output voltage. For switching
temperature and voltage characteristics. One may be regulators operating at low duty cycles (i.e. low output
tempted to use Z5U and Y5V ceramic capacitors for output voltage to input voltage conversion ratios), it is bene ? cial
? ltering because of their high capacitance density and to use rectifying diodes with somewhat higher average cur-
small sizes. However these types of capacitors have high rent ratings (thus lower forward voltages). This is because
temperature and high voltage coef ? cients. For example, the diode conduction interval is much longer than that of
the capacitance of a Z5U capacitor can drop below 60% the transistor. Converter ef ? ciency will be improved if the
of its room temperature value at –25°C and 90°C. X5R voltage drop across the diode is lower.
ceramic capacitors, which have stable temperature and
voltage coef ? cients, are the preferred type.
The rectifying diodes should be placed close to the SW
pin of the SC4503 to minimize ringing due to trace induc-
The diode current waveform in Figure 4 is discontinuous tance. Surface-mount equivalents of 1N5817 and 1N5818,
with high ripple-content. Unlike a buck converter in which MBRM120, MBR0520L, ZHCS400, 10BQ015 and equiva-
the inductor ripple current ? I L determines the output ripple lent are suitable.
voltage. The output ripple voltage of a boost regulator is
much higher and is determined by the absolute inductor Shutdown and Soft-Start
current. Decreasing the inductor ripple current does not
reduce the output ripple voltage appreciably. The current
The shutdown ( SHDN SS ) pin is a dual function pin. When
flowing in the output filter capacitor is the difference driven from a logic gate with V OH >2V, the SHDN SS pin
between the diode current and the output current. This functions as an on/off input to the SC4503. When the
capacitor current has a RMS value of:
shutdown pin is below 2V, it clamps the error ampli ? er
output to
and reduces the switch current limit.
?
(12)
Connecting R SS and C SS to the SHDN SS pin (Figure 5) slows
the voltage rise at the pin during start-up. This forces the
peak inductor current (hence the input current) to follow a
If a tantalum capacitor is used, then its ripple current rating slow ramp, thus achieving soft-start.
in addition to its ESR will need to be considered.
? 2007 Semtech Corp.
11
www.semtech.com
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