ADM1041-EVAL Analog Devices Inc, ADM1041-EVAL Datasheet - Page 21

• Manufacturer Part Number: ADM1041-EVAL
• Description: BOARD EVALUATION ADM1041
• Manufacturer: Analog Devices Inc

Specifications of ADM1041-EVAL

• Main Purpose: AC/DC, Secondary Side
• Outputs And Type: 1, Non-Isolated
• Power - Output: 24W
• Voltage - Output: 12V
• Current - Output: 2A
• Voltage - Input: 85 ~ 132VAC
• Board Type: Fully Populated
• Utilized Ic / Part: ADM1041
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Frequency - Switching: -
• Regulator Topology: -
• Lead Free Status / Rohs Status: Not Compliant

2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
220
200
180
160
140
120
100
70
60
50
40
30
20
10
80
60
40
20
0
0
0
0
1
1
Figure 12. C
10
10
Figure 11. V
100
100
1
Figure 10. Current Limit
CMP
and S
1k
1k
CMP
VOLTAGE (V)
BANDWIDTH
BANDWIDTH
Transconductance
CMP
10k
10k
2
Transconductance
100k
100k
1M
1M
3
10M
10M
100M
100M
4
Rev. A | Page 21 of 64
VOLTAGE ERROR AMPLIFIER
This is a high gain transconductance amplifier that takes its
input from the load voltage trim stage described previously. The
amplifier requires only the output pin for loop compensation,
which typically consists of a series RC network-to-common. A
parallel resistor may be added to common to reduce the open-
loop gain and thereby provide some output voltage droop as
output current increases. The output of the amplifier is typically
connected to an emitter follower that drives an optocoupler,
which in turn controls the duty of the primary side PWM. The
emitter follower should have a high gain to minimize loading
effects on the amplifier. Alternatively, an op amp voltage
follower may be used. See Figure 9, Figure 10, and Figure 11.
MAIN VOLTAGE REFERENCE
A 1.5 V reference is connected to the inverting input of the
voltage error amplifier. This 1.5 V reference is the output voltage
of the soft-start circuit. Under closed-loop conditions, the
voltage at the noninverting input is also controlled to 1.5 V.
During start-up, the output voltage should be ramped up in a
linear fashion at a rate that is independent of the load current.
This is achieved by digitally ramping up the reference voltage by
using a counter and a DAC. The ramp rate is configurable via
the SMBus. See Figure 14.
CURRENT SENSE AMPLIFIER
This is a two-stage differential amplifier that achieves low offset
and accuracy. The amplifier has the option to be chopped to
reduce offset or left as a linear amplifier without chopping.
Refer to the Register Listing for more details. Its gain may be
selected from three ranges. It is followed by a trim stage and
then by a low gain buffer stage that can be configured with a
gain of 1.0 or 2.1. The result is a total of six overlapping gain
ranges (65 to 230), one of which must be selected via the SMBus.
This gives ample adjustment to compensate for the poor initial
tolerance of the resistance wires typically used for current
sensing. It also allows selecting a higher sensitivity for better
efficiency or a lower sensitivity for better accuracy (lower offset).
The amplifier offset voltage is trimmed to zero in a once-off
operation via the SMBus and uses a voltage controlled current
source at the output of the first gain stage. A second controlled
current source is used to trim out the additional offset due to
the mismatch of the external divider resistors. This offset trim is
dynamically adjusted according to the common-mode voltage
present at the top of the voltage dividers. Six ranges are selectable
according to the magnitude and polarity of this offset component.
Because the offset compensation circuit itself has some inaccu-
racies, the best overall current sense accuracy is obtained by
using more closely matched external dividers and then selecting
a low compensation range. See Figure 14.
ADM1041