SC1189 查看數據表（PDF） - Semtech Corporation
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SC1189 Datasheet PDF : 16 Pages
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SC1189
POWER MANAGEMENT
FOLDBACK CURRENT LIMITING
The SC1189 implements a “Hard Current Limit”
M
=
IOLIM
IOS
overcurrent protection for the switching supply output. In
a short circuit condition, this will lead to higher than nor-
mal power dissipation in the bottom side FETs. If this is
problematic, foldback current limiting can be easily and
inexpensively implemented to drastically reduce dissipa-
3) Choose VB and calculate RB
RB
=
M ⋅ VB ⋅ RD ⋅ RF
(M − 1)VCS(RD + RF )
4) Calculate the ratio of the input divider
tion during output short circuit
VIN
RA
RB
RC
D1
To CS+
To CS-
VCS
RC
(RA + RC )
=
VB
+
VCS
VIN
+
VF
where VF = Forward Voltage drop of diode (≈ 0.6V)
Choose RC<RB/10 and calculate RA
1N4148
SHORT CIRCUIT PROTECTION - LINEARS
RD
RF
The Short circuit feature on the linear controllers is imple-
L1
RS
mented by using the Rds(on) of the FETs. As output cur-
VO
rent increases, the regulation loop maintains the output
voltage by turning the FET on more and more. Eventually,
Output Current Path
as the Rds(on) limit is reached, the FET will be unably to
Foldback current limit components
turn on more fully, and output voltage will start to fall.
When the output voltage falls to approximately 50% of
nominal, the LDO controller is latched off, setting output
voltage to 0. Power must be cycled to reset the latch.
To prevent false latching due to capacitor inrush currents
1
or low supply rails, the current limit latch is initially dis-
Breakpoint
abled. It is enabled at a preset time (nominally 2mS) after
VB
both the LDOV and LDOEN rails rise above their lockout
points.
To be most effective, the linear FET Rds(on) should not be
selected artificially low, the FET should be chosen so that,
0
0
IOS
at maximum required current, it is almost fully turned on.
IO1LIM
If, for example, a linear supply of 1.5V at 4A is required
from a 3.3V ± 5% rail, max allowable Rds(on) would be.
Foldback current limit characteristics
Rds(on)max = (0.95*3.3-1.5)/4 » 400mΩ
To allow for temperature effects 200mΩ would be a suit-
able room temperature maximum, allowing a peak short
For a complete design procedure for foldback current lim- circuit current of approximately 15A for a short time be-
iting see Application Note AN01-2, “Foldback Current fore shutdown.
Limit”. An abbreviated procedure is given below.
1) Choose values for IOLIM and RS and calculate the ratio
RF
RD + RF
=
VCS
IOLIM ⋅ RS
If this ratio > 1, the value of RS or IOLIM must be increased.
Then let RF=1kW and calculate RD.
2) Choose a short circuit current (IOS) and calculate M, do
not be too agressive with M, a value between 2 and 3
should be sufficient. Choosing too low a value for IOS will
result in a high value for M and may cause startup prob-
lems due to insufficient current.
2007 Semtech Corp.
11
www.semtech.com
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