LT1513/LT1513-2
U
W U U
APPLICATIONS INFORMATION
problem, and indeed small signal loop stability can be
excellent even in the presence of subharmonic switching.
TheprimaryissuewithsubharmonicsisthepresenceofEMI
at frequencies below 500kHz.
(from the Electrical Characteristics). Amplifier output resis-
tance is modeled with a 330k resistor. The power stage
(modulatorsection)oftheLT1513ismodeledasatranscon-
ductance whose value is 4(VIN)/(VIN + VBAT). This is a very
simplifiedmodeloftheactualpowerstage,butitissufficient
when the unity-gain frequency of the loop is low compared
totheswitchingfrequency.Theoutputfiltercapacitormodel
includes its ESR (RCAP). A series resistance (RBAT) is also
assigned to the battery model.
Constant-Current Mode Loop Stability
The LT1513 is normally very stable when operating in con-
stant-currentmode(seeFigure7), buttherearecertaincon-
ditions which may create instabilities. The combination of
highervaluecurrentsenseresistors(lowprogrammedcharg-
ingcurrent),higherinputvoltages,andtheadditionofaloop
compensation resistor (R5) on the VC pin may create an un-
stable current mode loop. (A resistor is sometimes added in
series with C5 to improve loop phase margin when the loop
is operating in voltage mode.) Instability results
because loop gain is too high in the 50kHz to 150kHz region
where excess phase occurs in the current sensing amplifier
and the modulator. The IFBA amplifier (gain of –12.5) has a
pole at approximately 150kHz. The modulator section con-
sisting of the current comparator, the power switch and the
magnetics, has a pole at approximately 50kHz when the
coupledinductorvalueis10µH.Higherinductancewillreduce
thepolefrequencyproportionally.Thedesignprocedurepre-
sented here is to roll off the loop to unity-gain at a frequency
of 25kHz or lower to avoid these excess phase regions.
Analysis of this loop normally shows an extremely stable
system for all conditions, even with 0Ω for R5. The one
condition which can cause reduced phase margin is with a
very large battery resistance (>5Ω), or with the battery
replaced with a resistive load. The addition of R5 gives good
phase margin even under these unusual conditions. R5
should not be increased above 330Ω without checking for
twopossibleproblems. Thefirstisinstabilityintheconstant
current region (see Constant-Current Mode Loop Stability),
andthesecondissubharmonicswitchingwhereswitchduty
cycle varies from cycle to cycle. This duty cycle instability is
caused by excess switching frequency ripple voltage on the
VC pin. Normally this ripple is very low because of the
filtering effect of C5, but large values of R5 can allow high
rippleontheVC pin.Normalloopanalysisdoesnotshowthis
I
P
V1
MODULATOR SECTION
4(V1)(V
C
)
IN
P
3pF
R **
P
I
P
=
V
IN
+ V
1M
BAT
FB
R4
24Ω
I
FB
–
+
R
A
100k
–
+
I
FBA
V
C
EA
R3
0.1Ω
R5
C
A
10pF
C4
0.22µF
VOLTAGE
GAIN = 12
R
330Ω
G
g
m
330k
C5
0.1µF
1.245V
1500µmho
THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN
THE CURRENT AMPLIFIER HAS A FIXED VOLTAGE GAIN OF 12.
CONSTANT-CURRENT MODE. RESISTOR AND CAPACITOR ITS PHASE DELAY IS MODELED WITH R AND C .
A
A
NUMBERS CORRESPOND TO THOSE USED IN FIGURE 1.
AND C MODEL THE PHASE DELAY IN THE PowerPath.
C3 IS 3pF FOR A 10µH INDUCTOR. IT SHOULD BE SCALED
PROPORTIONALLY FOR OTHER INDUCTOR VALUES (6pF
FOR 20µH). THE PowerPath IS A TRANSCONDUCTANCE
WHOSE GAIN IS A FUNCTION OF INPUT AND BATTERY
VOLTAGE AS SHOWN.
THE ERROR AMPLIFIER HAS A TRANSCONDUCTANCE OF
1500µmho AND AN INTERNAL OUTPUT SHUNT RESISTANCE OF
330k.
R
P
P
AS SHOWN, THIS LOOP HAS A UNITY-GAIN FREQUENCY OF
ABOUT 27kHz. R5 IS NOT USED IN ALL APPLICATIONS, BUT IT
GIVES BETTER PHASE MARGIN IN CONSTANT VOLTAGE MODE.
1513 F07
Figure 7. Constant-Current Small-Signal Model
11
Linear [ Linear ]
