參數(shù)資料
型號(hào)： SC1404_05
廠商： Semtech Corporation
英文描述： Differential Input, Dual, Simultaneous Sampling, 4.25 MSPS, 14-Bit, SAR ADC, 16-TSSOP, Ind Temp
中文描述： 移動(dòng)多輸出PWM控制器虛擬電流SenseTM
文件頁(yè)數(shù)： 16/25頁(yè)
文件大?。?/td> 472K
代理商： SC1404_05
16
2004 Semtech Corp.
www.semtech.com
SC1404
PRELIMINARY
POWER MANAGEMENT
Typical Characteristics
Input ripple current can be calculated from the following equations.
3V
3.3V/V
D3
=
duty
5V
5V/V
D5
=
cycle
cycle
duty
IN
=
IN
=
current
current
load
load
DC
DC
3V
5V
I3
=
I5
=
D
OVL
= overlapping duty cycle of the 3V and 5V pulses
(varies according to input voltage)
V
9.6V
for
D
≤
=
for
0.41)
(D5
D
OVL
=
V
for
0.36)
(D5
D
OVL
=
IN
6.7V
OVL
9.6V
<
V
IN
≤
6.7
IN
<
current
input
DC
D
Average
3
3
D
I
I
IN
=
I
IN
5
5
I
+
=
IN
SW_RMS
V
from
drawn
+
current
+
RMS
I
=
=
I5
I3
D
2
I5
D5
I3
D3
I
OVL
2
2
SW_RMS
2
2
2
IN_AVE
SW_RMS
RMS_CAP
I
I
I
+
=
The worst-case ripple current varies by application. For the case
6A load on both outputs, the worst-case ripple occurs at Vin =
7.5V, and the rms capacitor current is 4.2A. The reference design
uses 4 paralleled ceramic capacitors, (Murata GRM32NF51E106Z,
10 uF 25V, size 1210). Each capacitor is rated at 2.2A.
Choosing Synchronous mosfet and Schottky Diode
Since this is a buck topology, the voltage and current ratings of the
synchronous mosfet are the same as the main switching mosfet.
It makes sense cost- and volume-wise to use the same mosfet for
the main switch as for the synchronous mosfet. Therefore, IRF7413
is used again in the design for synchronous mosfet.
To improve overall efficiency, an external Schottky diode is used in
parallel with the low side mosfet. The freewheeling current enters
the Schottky diode instead of the inefficient body diode of the
synchronous mosfet. It is really important when laying out the
board to place the synchronous mosfet and Schottky diode close
to each other to reduce the current ramp-up and ramp-down time
due to parasitic inductance between the channel of the mosfet
and the Schottky diode. The current rating of the Schottky diode
can be determined by the following equation:
0.2A
TS
100n
LOAD
I
IF_AVG
=
=
where 100nsec is the estimated time between the mosfet turn-
off and the Schottky diode turn-on and Ts = 3.33uS.A Schottky
diode with a forward current of 0.5A is sufficient for this design.
Operation below 6V input
The SC1404 will operate below 6V input voltage with careful
design, but there are limitations. The first limitation is the
maximum available duty cycle from the SC1404, which limits
the obtainable output voltage. The design should minimize all
circuit losses through the system in order to deliver maximum
power to the output.
A second limitation with operation below 6V is transient
response. When load current increases rapidly, the output
voltage drops slightly; the feedback loop normally increases duty
cycle briefly to bring the output voltage back up. If duty cycle is
already near the maximum limit, the duty cycle cannot increase
enough to meet the demand, and the output voltage sags more
than normal. This problem can not be solved by changing the
feedback compensation, it is a function of the input voltage,
duty cycle, and inductor and capacitor values.
If an application requires 5V output from an input voltage below
6V, the following guidelines should be used:
1 - Set the switching frequency to 200 kHz (Tie SYNC to
ground). This increases the maximum duty cycle
compared to 300 kHz operation.
2 - Minimize the resistance in the power train. Select
mosfets, inductor, and current sense resistor to provide
the lowest resistance as is practical.
3 - Minimize the pcb resistance for all traces carrying
high current. This includes traces to the input
capacitors, mosfetS and diodes, inductor, current sense
resistor, and output capacitor.
4 - Minimize the resistance between the SC1404
circuit and the power source (battery, battery charger,
AC adaptor).
5 - Use low ESR capacitors on the input to prevent the
input voltage dropping during on-time.
6 - If large load transients are expected, high
capacitance and low ESR capacitors should be used on
both the input and output.
相關(guān)PDF資料
PDF描述
SC1404 Mobile Multi-Output PWM Controller with Virtual Current Sense
SC1404ISSTR Mobile Multi-Output PWM Controller with Virtual Current Sense
SC1404ITSTR Mobile Multi-Output PWM Controller with Virtual Current Sense
SC1405B HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER
SC1405BTS.TR High Speed Synchronous Power MOSFET Smart Driver
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
SC1404ISSTR 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:Mobile Multi-Output PWM Controller with Virtual Current SenseTM
SC1404ITSTR 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:Mobile Multi-Output PWM Controller with Virtual Current SenseTM
SC1405 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER
SC1405B 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER
SC1405BTS.TR 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:High Speed Synchronous Power MOSFET Smart Driver
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