9

UCC2956

UCC3956

APPLICATION INFORMATION (cont.)

vides a gain of 5, an inverting stage adds a gain of 1.5,

and the modulator has a gain of 0.64; adding a fixed gain

of 4.8 to the power stage. The current amplifier’s gain be-

parallel combination of RG1 and RG2 times the resistive

divider RG2/(RG1+RG2), simplifying to:

GCA = RF4

RG1

RF4 is selected to be 15k, resulting in a 10kHz crossover

frequency. Once RF4 is determined, CF3 and CF4 can

CF3 =

1

2• • fOSC• RF4

π

CF4 =

10

2• • fOSC• RF4

π

In the applications circuit, a value of 100pF is used for

CF3 and 1.0nF is used for CF4. Figure 4a shows the

power stage gain and feedback network gain for the cur-

rent loop. Figure 4b shows the total open loop gain and

phase.

Adding the Voltage Control Loop

The voltage loop comes into regulation during the over-

charge period of operation. The output of the voltage am-

plifier begins to decrease, demanding less current to the

battery. With the current loop closed, the power stage

the crossover frequency (10kHz). In order to avoid inter-

actions with the current loop, the voltage loop will cross

unity at 2kHz. The voltage loop is attenuated by the di-

vider RG1/(RG1+RG2). A single pole network is added

to the voltage amplifier, giving a high gain at DC. Refer-

ring to Figure 3, the voltage amplifier gain is equal to the

impedance of CF1 divided by RS1. A 2.2nF capacitor will

give a total crossover frequency near 2kHz. Figure 5a

shows the gain of the power and feedback stages for the

voltage loop. Figure 5b shows the total gain and phase of

the voltage loop.

-60

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0

20

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60

80

10

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10000

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1000000

FREQUENCY

POWER STAGE

GAIN

FEEDBACK

GAIN

Figure 5a. Voltage Loop Power Stage Gain

-150

-100

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0

50

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FREQUENCY

TOTAL LOOP GAIN

TOTAL PHASE

Figure 5b. Voltage Loop Total Gain and Phase

-80

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0

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100

120

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FREQUENCY

TOTAL LOOP GAIN

TOTAL PHASE

Figure 4b. Current Loop Total Gain and Phase

UNITRODE CORPORATION

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