欢迎访问ic37.com |
会员登录 免费注册
发布采购

LT1513CR 参数 Datasheet PDF下载

LT1513CR图片预览
型号: LT1513CR
PDF下载: 下载PDF文件 查看货源
内容描述: SEPIC Constant-或可编程电流/恒定电压电池充电器 [SEPIC Constant- or Programmable-Current/ Constant-Voltage Battery Charger]
分类和应用: 稳压器开关式稳压器或控制器电源电路电池开关式控制器
文件页数/大小: 16 页 / 303 K
品牌: Linear [ Linear ]
 浏览型号LT1513CR的Datasheet PDF文件第7页浏览型号LT1513CR的Datasheet PDF文件第8页浏览型号LT1513CR的Datasheet PDF文件第9页浏览型号LT1513CR的Datasheet PDF文件第10页浏览型号LT1513CR的Datasheet PDF文件第12页浏览型号LT1513CR的Datasheet PDF文件第13页浏览型号LT1513CR的Datasheet PDF文件第14页浏览型号LT1513CR的Datasheet PDF文件第15页  
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 0for R5. The one  
condition which can cause reduced phase margin is with a  
very large battery resistance (>5), 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 330without 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  
24  
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