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参数资料
| 型号： | SC4503WLTRT |
| 厂商： | Semtech |
| 文件页数： | 13/22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.)
circuit from the driving logic gate during fault condition.
Output ? lter pole, ω
= ?
= ?
,
In Figure 5(f) the shutdown pin is driven from a logic gate
whose V OH is higher than the supply voltage to the SC4503.
The diode clamps the maximum shutdown pin voltage to
Compensating zero, ω
= ?
and
IN
POWER
OUT
OUT
-
one diode voltage above the input power supply.
During soft-start, C SS is charged by the difference between
the R SS current and the shutdown pin current, . In
steady state, the voltage drop across R SS reduces the shut-
down pin voltage according to the following equation:
= ? (14)
In order for the SC4503 to achieve its rated switch current,
must be greater than 2V in steady state. This
puts an upper limit on R SS for a given enable voltage V EN (=
voltage applied to R SS ). The maximum speci ? ed is
50 μ A with = (see “Electrical Characteristics”).
Right half plane (RHP) zero, ω
V
STAGE
FB
COMP
Gm
+
R C
1.252V
R O
C C
VOLTAGE
REFERENCE
=
C4
( ? )
R1
R2
.
I
ESR
C2
R
V
The largest R SS can be found using (14):
R O is the equivalent output resistance of the error amplifier
<
μ
?
Figure 6. Simpli ? ed Equivalent Model of a Boost
Converter
ω
μ μ
If the enable signal is less than 2V, then the interfacing
options shown in Figures 5(d) and 5(e) will be preferred. The
methods shown in Figures 5(a) and 5(c) can still be used
however the switch current limit will be reduced. Variations
of and switch current limit with SHDN SS pin voltage
and temperature are shown in the “Typical Characteristics”.
Shutdown pin current decreases as temperature increases.
Switch current limit at a given also decreases as
temperature rises. Lower shutdown pin current ? owing
through R SS at high temperature results in higher shutdown
pin voltage. However reduction in switch current limit (at
a given ) at high temperature is the dominant
effect.
Feed-Forward Compensation
Figure 6 shows the equivalent circuit of a boost converter.
Important poles and zeros of the overall loop response
are:
The poles p 1 , p 2 and the RHP zero z 2 all increase phase
shift in the loop response. For stable operation, the over-
all loop gain should cross 0dB with -20dB/decade slope.
Due to the presence of the RHP zero, the 0dB crossover
frequency should not be more than . The internal
compensating zero z 1 provides phase boost beyond p 2 . In
general the converter is more stable with widely spaced
? lter pole p 2 and the RHP zero z 2 . The RHP zero moves to
low frequency when either the duty-cycle D or the output
current I OUT increases. It is bene ? cial to use small inductors
and larger output capacitors especially when operating at
high ratios.
A feed-forward capacitor C 4 is needed for stability. The value
of C 4 can be determined empirically by observing the induc-
tor current and the output voltage during load transient.
Starting with a value between and , C 4 is
Low frequency integrator pole, ω
=?
,
adjusted until there is no excessive ringing or overshoot in
inductor current and output voltage during load transient.
Sizing the inductor such that its ripple current is about 0.5A
also improves phase margin and transient response.
? 2007 Semtech Corp.
13
www.semtech.com
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