SC5010H View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5010H
Semtech Corporation
'SC5010H' PDF : 36 Pages View PDF
SC5010H
Components Selection (continued)
The GD (gate driver) on SC5010H provides 1A (peak)
current driving capability which is suitable for most
MOSFETs for high frequency operation. The average
current required to drive the MOSFET is given by the fol-
lowing equation.
IGATE = QG x fSW
QG — Gate charge
The RDS(ON) and its RMS current IS_RMS of the power MOSFET
will generate the conduction loss using the following
equation.
PCOND
=
I2
S_RMS
x
RDS(on)
The MOSFET’s switch loss can be calculated using the fol-
lowing equation.
PSW = ½ x VIN x IL_PEAK x fSW x (TON + TOFF)
Where TON and TOFF are the MOSFET’s on and off time
and they can be estimated by the following
equations.
7
21
W
U
9
4
JG
5
SODWH D X
J
7 2))
WI
4
9 SODWH D X
JG
5
J
Where tr , tf , Qgd and Vplateau can usually be found from data-
sheet of the selected MOSFET. Rg is the resistance of the
optional resistor connected in series on the gate of the
MOSFET.
Current Sensing Resistor Selection
The switch current is sensed via the current sensing resis-
tor, RSNS. The sensed voltage at this pin is used to set the
peak switch current limit and also used for steady state
regulation of the inductor current. The current limit com-
parator has a trip voltage of 0.4V. RSNS value is chosen to
set the peak inductor and switch current using the follow-
ing equation.
ISW(Peak) = 0.4/RSNS
The power dissipation in RSNS can be calculated using the
following equations.
PR_SNS
=
I2
RMS
x
RSNS
IRMS = D x [IO/(1-D)]2
IO = Output DC Current, D = Duty Cycle
For the typical application circuit shown in the Detailed
Application Circuit (page 21), the power dissipation on the
sensing resistor is shown by the following equations.
Assuming VIN(Min) = 6.5V and VOUT = 30V, thus D = 78.3%,
IO = 50mA/string x 6 string = 300 (mA) = 0.3 (A)
IRMS = D x [IO/(1-D)]2 = 78.3% x [0.3A/(1-78.3%)]2
= 1.5 (A)
PR_SNS = 1.52 x 0.1 = 0.22 (W)
For this example, a 0.1 Ω 1% thick-film chip resistor rated
at 0.25W or higher power can be used.
PLL Filter Component Selection
The Detailed Application Circuit on page 21 shows the
optimal R/C filter components for the PLL compensation.
These are optimized for internal 1MHz switching fre-
quency. Please contact Semtech application group if a
different switching frequency is selected.
Isolation MOSFET Selection
The external p-channel MOSFET provides load disconnec-
tion during shutdown or fault condition. Select a MOSFET
with low RDS(on) to limit the power loss.
In order to implement inrush current limiting, a 10nF
capacitor is connected between the gate and source ter-
minal of the MOSFET.
If isolation is not required, then the DRVP pin can be left
floating. Connect the VIN directly to the system input
supply or, to the VCC (5V) supply. Do not leave the VIN pin
floating.
26
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