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參數(shù)資料
| 型號： | SC1404ITSTR |
| 廠商： | Semtech Corporation |
| 英文描述： | Mobile Multi-Output PWM Controller with Virtual Current Sense |
| 中文描述： | 移動多輸出與虛擬PWM控制器電流檢測 |
| 文件頁數(shù)： | 17/27頁 |
| 文件大小： | 515K |
| 代理商： | SC1404ITSTR |
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17
2003 Semtech Corp.
www.semtech.com
SC1404
POWER MANAGEMENT
Typical Characteristics
Choosing Synchronous MOSFET and Schottky diode
Since this is a buck topology, the voltage and current ratings of the
synchronous MOSFET are similar to the high-side MOSFET. It makes
sense cost-volume-wise to use the same MOSFET for both the
main switch and synchronous MOSFET. Therefore, STS12NF30L
is used again in the design for synchronous MOSFET. To improve
overall efficiency, an external schottky diode is used in parallel to
the synchronous MOSFET. The freewheeling current goes into the
schottky diode instead of the body diode of the synchronous
MOSFET, which usually has very high forward drop and slow tran-
sient behavior. It is important when laying out the board to place
both 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.
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
GND). 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.
A
2
T
n
100
I
I
S
LOAD
AVG
_
F
=
=
Vin < 6.7 volts: 3.3V turn-on leads 5V turn-on by 64% of the
period. The 5V turn-on is delayed slightly more to add
separation between the 3V turn-off and 5V turn-on. This leads
to more overlap, but at this point overlap is unavoidable.
Input ripple current calculations: The following equations
provide quick approximations for input ripple current:
D3 = 3.3V duty cycle = 3.3/Vin
D5 = 5V duty cycle = 5/Vin
I3 = 3.3V load current
I5 = 5V load current
Dovl = overlapping duty cycle of the 3V and 5V pulses, which
varies according to input voltage:
Vin > 9.6V:
9.6V > Vin > 6.7V:
6.7V > Vin
Iin = D3 . I3 + D5 . I5 (average current drawn from Vin)
Dovl = 0
Dovl = D5 - 0.41
Dovl = D5 - 0.36
(Isw_rms)2 = Dovl . (I3 + I5)2 + (D3 - Dovl) . I32 +
(D5 -Dovl) . I52
Isw_rms = rms current flowing into 3V and 5V SMPS
Irms_cap =
Isw_rms
2
Iin
2
+
The worst-case ripple current varies by application. For the case
of I3 = I5 = 6A, the worst-case ripple occurs at Vin = 7.5V, at which
point the rms capacitor ripple current is 4.2 amps. To handle this
the reference design uses 4 paralleled ceramic capacitors, (Murata
GRM32NF51E106Z, 10 uF 25V, size 1210). Each capacitor is
rated at 2.2 Amps, allowing for derating at higher temperatures.
where 100nsec is the estimated time between the MOSFET turn-
ing off and the Schottky diode taking over and Ts = 3.33uS. There-
fore a Schottky diode with a forward current of 0.5A is sufficient
for this design.
Operation below 6V input
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| SC1405B | HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER |
| SC1405BTS.TR | High Speed Synchronous Power MOSFET Smart Driver |
| SC1405D | Differential Input, Dual, Simultaneous Sampling, 4.25 MSPS, 14-Bit, SAR ADC, 16-TSSOP, Auto Temp |
| SC1405DITSTRT | Differential Input, Dual, Simultaneous Sampling, 4.25 MSPS, 14-Bit, SAR ADC, 16-TSSOP, Auto Temp |
| SC1405 | HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| SC1405 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER |
| SC1405B | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:HIGH SPEED SYNCHRONOUS POWER MOSFET SMART DRIVER |
| SC1405BTS.TR | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Speed Synchronous Power MOSFET Smart Driver |
| SC1405CTS.TR | 制造商:Semtech Corporation 功能描述: |
| SC1405D | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Speed Synchronous Power MOSFET Smart Driver |
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