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| 型号： | SIC424CD-T1-GE3 |
| 厂商： | Vishay Siliconix |
| 文件页数： | 15/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 6A 44MLP-28 |
| 标准包装： | 1 |
| 系列： | microBUCK® |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.75 V ~ 5.5 V |
| 输入电压： | 3 V ~ 28 V |
| 频率 - 开关： | 200kHz ~ 1MHz |
| 电流 - 输出： | 6A |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 28-MLPQ（4x4） |
| 其它名称： | SIC424CD-T1-GE3DKR |
SiC414, SiC424
Vishay Siliconix
Using the On-chip LDO to Bias the SIC414/SIC424
The following steps must be followed when using the onchip
R tON =
1
25 pF x f sw
- 400 Ω x
V IN
V OUT
LDO to bias the device.
? Connect V5V to V LDO before enabling the LDO.
? Any external load on V LDO should not exceed 40 mA until
To select R tON , use the maximum value for V IN , and for t ON
use the value associated with maximum V IN .
the LDO voltage has reached 90 % of final value.
? Do not connect the EN pin directly to the V5V or any other
t O N =
V OUT
V I N MAX. x f S W
supply voltage if V OUT is greater than or equal to 4.5 V.
Many applications connect the EN pin to V5V and control the
on/off of the LDO and PWM simultaneously with the ENL pin.
This allows one signal to control both the bias and power
output of the SiC414 and SiC424. When V OUT > 4.5 V this
configuration can cause problems due to the parasitic diodes
in the LDO switchover circuitry. After the V OUT > 4.5 V PWM
output is up and running the switchover diodes can hold up
V5V > UVLO even if the ENL pin is grounded, turning off the
LDO. Operating in this way can potentially damage the part.
Design Procedure
When designing a switch mode power supply, the input
voltage range, load current, switching frequency, and
inductor ripple current must be specified.
The maximum input voltage (V INMAX ) is the highest specified
input voltage. The minimum input voltage (V INMIN ) is
determined by the lowest input voltage after evaluating the
voltage drops due to connectors, fuses, switches, and PCB
traces.
The following parameters define the design:
? Nominal output voltage (V OUT )
? Static or DC output tolerance
? Transient response
? Maximum load current (I OUT )
There are two values of load current to evaluate - continuous
load current and peak load current. Continuous load current
relates to thermal stresses which drive the selection of the
inductor and input capacitors. Peak load current determines
instantaneous component stresses and filtering
t ON = 303 ns at 13.2 V IN , 1 V OUT , 250 kHz
Substituting for R tON results in the following solution
R tON = 130.9 k ? , use R tON = 130 k ? .
Inductor Selection
In order to determine the inductance, the ripple current must
first be defined. Low inductor values result in smaller size but
create higher ripple current which can reduce efficiency.
Higher inductor values will reduce the ripple current/voltage
and for a given DC resistance are more efficient. However,
larger inductance translates directly into larger packages and
higher cost. Cost, size, output ripple, and efficiency are all
used in the selection process.
The ripple current will also set the boundary for power-save
operation. The switching will typically enter power-save
mode when the load current decreases to 1/2 of the ripple
current. For example, if ripple current is 4 A then Power-save
operation will typically start for loads less than 2 A. If ripple
current is set at 40 % of maximum load current, then power-
save will start for loads less than 20 % of maximum current.
The inductor value is typically selected to provide a ripple
current that is between 25 % to 50 % of the maximum load
current. This provides an optimal trade-off between cost,
efficiency, and transient performance.
During the DH on-time, voltage across the inductor is
(V IN - V OUT ). The equation for determining inductance is
shown next.
requirements such as inductor saturation, output capacitors,
and design of the current limit circuit.
The following values are used in this design:
L=
( V I N - V OUT ) x t O N
I RIPPLE
? V IN = 12 V ± 10 %
? V OUT = 1.5 V ± 4 %
? f SW = 250 kHz
? Load = 6 A maximum
Frequency Selection
Example
In this example, the inductor ripple current is set equal to
50 % of the maximum load current. Therefore ripple current
will be 50 % x 6 A or 3 A. To find the minimum inductance
needed, use the V IN and t ON values that correspond to
V INMAX.
Selection of the switching frequency requires making a
trade-off between the size and cost of the external filter
components (inductor and output capacitor) and the power
L=
( 13.2 V - 1 V ) x 31 8 ns
3A
= 1.26 μ H
conversion efficiency.
The desired switching frequency is 250 kHz which results
from using component selected for optimum size and cost.
A resistor (R tON ) is used to program the on-time (indirectly
setting the frequency) using the following equation.
A slightly larger value of 1.5 μH is selected. This will
decrease the maximum I RIPPLE to 2.53 A.
Note that the inductor must be rated for the maximum DC
load current plus 1/2 of the ripple current. The ripple current
under minimum V IN conditions is also checked using the
following equations.
Document Number: 63388
S13-0248-Rev. B, 04-Feb-13
For technical questions, contact: powerictechsupport@vishay.com
www.vishay.com
15
This document is subject to change without notice.
THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
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