SC2446AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2446AEVB Datasheet PDF : 27 Pages
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SC2446A
POWER MANAGEMENT
Application Information (Cont.)
It is noted that the DC value of VCs is independent of the
value of L, R and C . This means that, if only the average
s
s
load current information is needed (such as in average
current mode control), this current sensing method is
effective without time constant matching requirement.
In the current mode control as implemented in SC2446A,
the voltage ripple on C is critical for PWM operation. In
s
fact, the AC voltage ripple peak-to-peak value of VCs
(denoted as ∆VCs) directly effects the signal-to-noise ratio
of the PWM operation. In general, smaller ∆VCs leads to
lower signal-to-noise ratio and more noise sensitive
operation. Larger ∆VCs leads to more circuit (power stage)
parameter sensitive operation. A good engineering
compromise is to make
∆VCs~
R
L
δIo.
when the load is sourcing current from the converter and
ILMcn
75mV
= − RL
,
when the load is forcing current back to the input power
source. If RL = 1.8mΩ, then ILM = 27.8 / -41.7A. The circuit
in Figure 11 allows the user to scale the equivalent
current limit with the same RL.
In the following design steps, the capacitor CS in the
current sensing part is commonly selected in the range
of 22nF ~ 100nF.
The pre-requisite for such relation is the so called time
constant matching condition
L
RL
≈ RsCs.
For an example of application circuit, L = 1µH,
RL = 1.8mΩ, the time constant RsCs should be set as
555.6µs. If one selects Cs = 33nF, then Rs = 16.9 kΩ.
Vin
Cin
Q1
Vgs1
PN
iL(t) L
Rs
Q2
Vgs2
RL
Rs1
Cs
vC(t)
Vo
Cout Rload
Scaling the Current Limit
Over-current is handled differently in the SC2446A
depending on the direction of the inductor current. If the
differential sense voltage between CS+ and CS- exceeds
+50mV, the top MOSFET will be turned off and the bottom
MOSFET will be turned on to limit the inductor current.
This +50mV is the cycle-by-cycle peak current limit when
the load is drawing current from the converter. There is
no cycle-by-cycle current limit when the inductor current
flows in the reverse direction. If the voltage between
CS1+ and CS- falls below -75mV, the controller will
undergo overload shutdown and time-out with both the
top and the bottom MOSFETs shut off. (See the section
Overload Protection and Hiccup).
In the circuit of Figure 11, the equivalent inductor current
limits are set according to
ILMcp
=
50mV ,
RL
ISEN
+1
-2
Rs2
Rs3
Figure 11. Scaling the equivalent current limit.
a) When the required current limit value ILM is greater
than ILMcp, one just needs to remove Rs3, and solve the
following equations
(Rs
// Rs1)
Cs
=
L
RL
,
and
for
ILMRL
Rs1
Rs + Rs1
=
50mV,
Rs2 = Rs // Rs1 .
Rs,Rs1 and Rs2.
2005 Semtech Corp.
15
www.semtech.com
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