SC2677BEVB-2 データシートの表示（PDF） - Semtech Corporation
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SC2677BEVB-2
Semtech Corporation
SC2677BEVB-2 Datasheet PDF : 19 Pages
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SC2677B
POWER MANAGEMENT
Application Information(Cont.)
(R13+R19) vs.Oscillator Frequency
1000
900
800
700
600
500
400
300
4 6 8 10 12 14 16 18 20
(R13+R19) (kohm)
Vphasing vs Phase Shift
180
160
140
120
100
80
60
40
20
0
0.55
0.60
0.65 0.70 0.75 0.80
Vphasing (V)
0.85
0.90
ICC : Supply current for controller.
Q : Total gate charge of all selected MOSFETs.
GT
QG : Total gate charge of per selected MOSFETs.
FSW : Switching frequency.
N : Number of MOSFET.
It’s recommended that the below figure be performed to
ensure SC2677B under safe operation area.
QGT limitation (with loading)
620
580
540
500
460
420
SOA
380
340
300
260
220
180
SOA
140
100
60
SOA
20
150 200 250 300 350 400 450 500
Fsw(KHz)
5Vin
8.5Vin
12Vin
Over Current Protection
Current sense amplifiers sense the inductor DCR, and com-
pare with an internal OCP reference. As over current being
detected, the current sense amplifier will trip the peak
current limit on cycle-by-cycle basis. If the over current
condition sustains, and the output voltage drops below
75% of its nominal voltage level, the PWM will be disabled
and the power supply be latched off with short amount of
delay. The latch can be reset by power cycling.
Controller Power Dissipation
Controller power dissipation is generated by following
parameter; switching frequency, total gate charge of all
selected MOSFETs and supply voltage.
P = Vin * (ICC + QGT* FSW)
QGT = QG * N
Where
Vin : Supply voltage for controller and driving
MOSFET.
An example is shown below to demonstrate the
procedure introduced above.
Then
Vin =12V
Fsw =250KHz
N =4(number of MOSFET)
QGT = 108nC
QG = 27nC (per MOSFET)
Layout Guidelines
Power and signal traces must be kept separated for noise
considerations. Feedback, current sense traces and ana-
log ground should not cross any traces or planes carrying
high switching currents, such as in the input loop or the
phase node.
The input loop, consisting of the input capacitors and both
MOSFETs must be kept as small as possible. Since all of
the high switching currents occur in the input loop, the
enclosed loop area must be kept small to minimize induc-
tance and radiated and conducted noise emissions.
© 2009 Semtech Corp.
8
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