参数资料
| 型号： | SC4525CSETRT |
| 厂商： | Semtech |
| 文件页数： | 9/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3A 8SOIC |
| 标准包装： | 1 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1 V ~ 26.88 V |
| 输入电压： | 3 V ~ 28 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 300kHz ~ 1.3MHz |
| 电流 - 输出： | 3A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 105°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm Width）裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 8-SOIC-EP |
| 其它名称： | SC4525CSEDKR |
SC4525C
Applications Information
Operation
The SC4525C is a constant-frequency, peak current-mode,
step-down switching regulator with an integrated 28V,
3.9A power NPN transistor. Programmable switching
frequency makes the regulator design more flexible. With
the peak current-mode control, the double reactive poles
of the output LC filter are reduced to a single real pole by
the inner current loop. This simplifies loop compensation
and achieves fast transient response with a simple Type-2
compensation network.
As shown in Figure 2, the switch collector current is
sensed with an integrated 3.53m W sense resistor. The
sensed current is summed with a slope-compensating
ramp before it is compared with the transconductance
error amplifier (EA) output. The PWM comparator trip
point determines the switch turn-on pulse width. The
current-limit comparator ILIM turns off the power switch
when the sensed signal exceeds the 8mV current-limit
threshold.
Driving the base of the power transistor above the
input power supply rail minimizes the power transistor
saturation voltage and maximizes efficiency. An external
bootstrap circuit (formed by the capacitor C and the
diode D in Figure ) generates such a voltage at the BST
pin for driving the power transistor.
Shutdown and Soft-Start
The SS/EN pin is a multiple-function pin. An external
capacitor (4.7nF to 22nF) connected from the SS pin to
ground sets the soft-start and overload shutoff times of
the regulator (Figure 3). The effect of V SS/EN on the SC4525C
is summarized in Table .
shown in Figure 3). As the SS/EN voltage exceeds 0.4V,
the internal bias circuit of the SC4525C turns on and the
SC4525C draws 2mA from V IN . The .9μA charging current
turns off and the 2.4μA current source I C in Figure 3 slowly
charges the soft-start capacitor.
The error amplifier EA in Figure 2 has two non-inverting
inputs. The non-inverting input with the lower voltage
predominates. One of the non-inverting inputs is biased
to a precision V reference and the other non-inverting
input is tied to the output of the amplifier A . Amplifier A
produces an output V = 2(V SS/EN - .2V). V is zero and COMP
is forced low when V SS/EN is below .2V. During start up,
the effective non-inverting input of EA stays at zero until
the soft-start capacitor is charged above .2V. Once V SS/EN
exceeds .2V, COMP is released. The regulator starts to
switch when V COMP rises above 0.4V. If the soft-start interval
is made sufficiently long, then the FB voltage (hence the
output voltage) will track V during start up. V SS/EN must be
at least .83V for the output to achieve regulation. Proper
soft-start prevents output overshoot. Current drawn from
the input supply is also well controlled.
Overload / Short-Circuit Protection
Table 2 lists various fault conditions and their
corresponding protection schemes in the SC4525C.
Table 2: Fault conditions and protections
Condition Cause of Fault Protective Action
Cycle-by-cycle limit at
IL>ILimit, V FB >0.8V Over current
programmed frequency
Cycle-by-cycle limit with
IL>ILimit, V FB <0.8V Over current
frequency foldback
VSS/EN Falling Persistent over current Shutdown, then retry
SS/EN<1.9V or short circuit (Hiccup)
Table 1: SS/EN operation modes
SS/EN
Mode
Supply Current
Tj>160C
Over temperature
Shutdown
<0.2V
Shutdown
18uA @ 5Vin
As summarized in Table , overload shutdown is disabled
Load dependent
0.4V to 1.2V Not switching 2mA
1.2V to 2.15V Switching & hiccup disabled
>2.15V Switching & hiccup armed
Pulling the SS/EN pin below 0.2V shuts off the regulator
and reduces the input supply current to 8μA (V IN = 5V).
When the SS/EN pin is released, the soft-start capacitor
during soft-start (V SS/EN <2. 5V). In Figure 3, the reset input
of the overload latch B 2 will remain high if the SS/EN
voltage is below 2. 5V. Once the soft-start capacitor is
charged above 2. 5V, the output of the Schmitt trigger
B goes high, the reset input of B 2 goes low and hiccup
is charged with an internal .9μA current source (not
9
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