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SC1403 Arkusz danych(PDF) 20 Page - Semtech Corporation

Numer części SC1403
Szczegółowy opis  Mobile Multi-Output PWM Controller with Virtual Current SenseTM
PDF  30 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1403 Arkusz danych(HTML) 20 Page - Semtech Corporation

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 2002 Semtech Corp.
www.semtech.com
SC1403
PRELIMINARY
POWER MANAGEMENT
PRELIMINARY
Ic a p
Iin
averag e
0
0
I3 in
I5 in
As the input voltage is reduced, the duty cycle of both converters
increases. For inputs less than 8.3 volts it is impossible to prevent
overlap when producing 3.3V and 5V outputs, regardless of the
phase relationship between the converters. This can be seen in
the following figure.
I3 in
I5 in
Iin
0
averag e
Ic a p
0
pe riod
ph ase lea d
From an input filter standpoint it is desirable to make the minimize
the overlap, but it is also desirable to keep the turn-on and turn-off
transitions of the two converters separated in time, otherwise the
two converters may affect each other due to switching noise. The
SC1403 implements this by changing the phase relationship
between the converter depending on the input voltage.
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Vin > 9.6V: 3.3V turn-on leads 5V turn-on by 41% of the switching
period. With Vin > 9.6V it is always possible to achieve no overlap,
which minimizes the input ripple current. At Vin = 9.6V there is no
overlap, but the 3.3V turn-on is nearing the 5V turn-off converter.
6.7 < Vin < 9.6V: 3.3V turn-on leads 5V turn-on by 59% of the
period. To prevent the 3V turn-on from coinciding with the 5V turn-
off (which could affect either output), the 5V pulse is delayed in
time slightly such that the 3V turn-on occurs before the 5V turn-off.
This creates a small overlap between the 3V turn-on and the 5V
turn-off, with a resulting slight increase in RMS input ripple, but
this is preferred since it greatly reduces noise problems caused by
simultaneous transitions. Note that at Vin = 6.7, the 3V turn-off is
nearing the 5V turn-on.
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:
Dovl = 0
9.6V > Vin > 6.7V:
Dovl = D5 - 0.41
6.7V > Vin
Dovl = D5 - 0.36
Irms_cap = Isw_rms2 Iin2
+
Iin = D3 . I3 + D5 . I5 (average current drawn from Vin)
Isw_rms = rms current flowing into 3V and 5V SMPS
(Isw_rms)2 = Dovl . (I3 + I5)2 + (D3 - Dovl) . I32 +
(D5 -Dovl) . I52
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.
Out-of-phase Switching



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