ADP3810/ADP3811

–13–

REV. 0

of these stages is the value of GM times the load resistance. At the

COMP pin, the internal load resistance, R5, is typically 400 k

Ω.

The optocoupler gain is the typical value taken from the

MOC8103 data sheet. The voltage error amplifier gain is due

put of the internal amplifier, as labeled in Figure 31. The actual

op amp is assumed to have sufficient open-loop gain and band-

width compared to the system bandwidth; as a result, it can be

considered an ideal transimpedance amplifier. The pole created

quency to not affect the compensation.

The power stage gain equation is linearized based on primary

side current mode control with the flyback transformer operat-

ing with discontinuous inductor current.

∆I

Since the minimum current is 0.0 A,

∆I

maximum change in control voltage is set by internal circuitry

within the 3845 to

∆V

ferent for the voltage and current loop cases. For the voltage

loop without the battery, the effective load is R4, but for the

voltage limit has not been reached, so the maximum output

voltage is equal to the maximum output current times the load

resistor. Thus, the entire expression under the square root re-

duces to 1.0. Substituting these values into the general equation

for the power stage yields the specific gain values shown for

GM4.

When calculating the loop gain for the voltage loop and the cur-

rent loop, there are two main differences. First, GM2 applies

only to the voltage loop, and GM1 applies only to the current

loop. Use the appropriate GM input stage for the particular

loop calculations. Second, there are three battery conditions to

consider. For the current loop, the battery is present and un-

charged. Thus, the battery is modeled as a very large capaci-

tance (greater than 1 Farad). For the voltage loop, the battery is

amplifiers are represented by voltage controlled current sources,

the optocoupler by a current controlled current source, and the

error amplifier by a voltage controlled voltage source.

Design Criteria

Charging a 6 cell NiCad battery.

× 1.67 V = 10 V

Max Charge Current:

Ω

Pick a value for R1:

R1 = 80.6 k

Ω

Calculated Current Sense

Resistor:

Ω

Calculated Voltage Sense

Divider:

R2 = 20 k

Ω

Output Filter Cap:

Ω)

2nd Filter Cap:

µF (ESR = 0.2 Ω)

Gain of Each Block

ADP3810/ADP3811

GM1 = 8.3 mA/V

ADP3810/ADP3811

GM2 = 2.1 mA/V

ADP3810/ADP3811

Output Buffer:

GM3 = 6 mA/V

Optocoupler:

Voltage Error Amplifier:

∆V

Power Stage (General):

GM4 =

∆I

OMAX

∆V

C









Power Stage

(Voltage Loop):

GM4 = 0.091 A/V

Power Stage

(Current Loop):

GM4 = 1.0 A/V

The gains for the ADP3810/ADP3811 GM amplifiers are based

on typical measurements of the IC’s open-loop gain, and they

are expressed in units of milliamps per volt. The dc voltage gain

OPTO COUPLER

1nF

3.3k

Ω

1nF

R3

20k

Ω

0.25

Ω

0.2µF

300

Ω

R1

80.6k

Ω

R4

1.2k

Ω

BATTERY

COMP

1.0V

80k

Ω

GM3

6mA/V

GM1

8.3mA/V

2.0V

GM2

2.1mA/V

ADP3810/

ADP3811

COMP

R5

400k

Ω

OUT

R6

200

Ω

+5V

R

VOLTAGE ERROR

AMPLIFIER

∆V

C

R2

20k

Ω

GM4

POWER

STAGE

220µF

2R

Figure 31. Block Diagram of the Linearized Feedback Model