Datasheet

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R
F1
+
L
R
LDC(Tbase) C
FLT
a
(W)
(13)
R
LDC(T1)
+ R
LDC(Tbase)
R
REL(effT1)
(W)
(14)
R
LDC(T2)
+ R
LDC(Tbase)
R
REL(effT2)
(W)
(15)
R
THE(T1)
+
a R
LDC(Tbase)
R
F1
R
LDC(T1)
* a R
LDC(Tbase)
(W)
(16)
R
THE(T2)
+
a R
LDC(Tbase)
R
F1
R
LDC(T2)
* a R
LDC(Tbase)
(W)
(17)
a + 1 *
R
NTC(T1)
* R
NTC(T2)
R
THE(T1)
* R
THE(T2)
(dimensionless)
(18)
b + R
NTC(T1)
) R
NTC(T2)
(W)
(19)
c + R
NTC(T1)
R
NTC(T2)
(W
2
)
(20)
R
F3
+
* b " b
2
* 4ac
Ǹ
2a
(W)
(21)
R
F2
+
R
THE(T1)
ǒ
R
F3
) R
NTC(T1)
Ǔ
* R
F3
R
NTC(T1)
R
F3
) R
NTC(T1)
(W)
(22)
Establishing Current Feedback
R
GM
+
3
43.443 gm
CSA
2
) 0.01543 gm
CSA
) 3.225 10
*6
(W)
(23)
TPS40100
SLUS601–MAY 2005
APPLICATION INFORMATION (continued)
where
• L is the value of the output inductance (H)
• C
FLT
is the value of the current sense filter capacitor (F)
•αis the attenuation ratio chosen from Equation 11
• R
THE(T1)
,R
THE(T2)
are the equivalent resistances of the R
THE
network at temperatures T1 and T2
• R
LDC(Tbase)
is the DC resistance of the inductor at temperature T
BASE
in Ω
• R
LDC(T1)
,R
LDC(T2)
are the inductor resistances at temperatures T1 and T2
• R
REL(effT1)
,R
REL(effT2)
, are the relative resistances of the inductor at T1 and T2 vs. Tbase
• R
NCT(T1)
,R
NTC(T2)
are the effective resistance of the NTC thermistor at temperatures T1 and T2
The amount of current feedback in a given application is programmable by the user. The amount of current
feedback used is intended to be just enough to reduce the Q of the output filter double pole. This allows design
of a converter control loop that is stable for a very wide range of output capacitance. Setting the current feedback
higher offers little real benefit and can actually degrade load transient response, as well as introduce pulse
skipping in the converter. The current feedback is adjusted by setting the gain of the current sense amplifier. The
amplifier is a transconductance type and its gain is a set by connecting a resistor from the GM pin to GND:
where
• R
GM
is the resistor that sets the gain of the amplifier (Ω)
• gm
CSA
is the gain of the current sense amplifier (S)
The value of the sense amplifier gain should be less than 1000 µS, and more than 250 µS, with the resulting
gain setting resistor greater than 50 kΩ. As a suggested starting point, set the gain of the current sense amplifier
to a nominal 280 µS with RGM of 279 kΩ. This value should accommodate most applications adequately.
Figure 5 shows the current sense amplifier gain setting resistance vs. the sense amplifier gain.
14