11

LT1513/LT1513-2

Figure 7. Constant-Current Small-Signal Model

problem, and indeed small signal loop stability can be

excellent even in the presence of subharmonic switching.

The primary issue with subharmonics is the presence of EMI

at frequencies below 500kHz.

Constant-Current Mode Loop Stability

The LT1513 is normally very stable when operating in con-

stant-current mode (see Figure 7), but there are certain con-

ditions which may create instabilities. The combination of

highervaluecurrentsenseresistors(lowprogrammedcharg-

ing current), higher input voltages, and the addition of a loop

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

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

coupledinductorvalueis10

µH.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.

(from the Electrical Characteristics). Amplifier output resis-

tance is modeled with a 330k resistor. The power stage

(modulator section) of the LT1513 is modeled as a transcon-

simplified model of the actual power stage, but it is sufficient

when the unity-gain frequency of the loop is low compared

to the switching frequency. The output filter capacitor model

assigned to the battery model.

Analysis of this loop normally shows an extremely stable

system for all conditions, even with 0

Ω for R5. The one

condition which can cause reduced phase margin is with a

very large battery resistance (> 5

Ω), 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 330

Ω without checking for

two possible problems. The first is instability in the constant

current region (see Constant-Current Mode Loop Stability),

and the second is subharmonic switching where switch duty

cycle varies from cycle to cycle. This duty cycle instability is

caused by excess switching frequency ripple voltage on the

filtering effect of C5, but large values of R5 can allow high

1M

1500

µmho

MODULATOR SECTION

V1

FB

1513 F07

1.245V

EA

VOLTAGE

GAIN = 12

330k

R5

330

Ω

C5

0.1

µF

10pF

100k

R4

24

Ω

R3

0.1

Ω

3pF

C4

0.22

µF

THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN

CONSTANT-CURRENT MODE. RESISTOR AND CAPACITOR

NUMBERS CORRESPOND TO THOSE USED IN FIGURE 1.

C3 IS 3pF FOR A 10

µH INDUCTOR. IT SHOULD BE SCALED

PROPORTIONALLY FOR OTHER INDUCTOR VALUES (6pF

FOR 20

µH). THE PowerPath IS A TRANSCONDUCTANCE

WHOSE GAIN IS A FUNCTION OF INPUT AND BATTERY

VOLTAGE AS SHOWN.

THE CURRENT AMPLIFIER HAS A FIXED VOLTAGE GAIN OF 12.

THE ERROR AMPLIFIER HAS A TRANSCONDUCTANCE OF

1500

µmho AND AN INTERNAL OUTPUT SHUNT RESISTANCE OF

330k.

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.