Datasheet

Data Sheet ADN8831
Rev. A | Page 17 of 20
MOSFET DRIVER AMPLIFIER
The ADN8831 has two separate MOSFET drivers: a switched
output or pulse-width modulated (PWM) amplifier, and a high
gain linear amplifier. Each amplifier has a pair of outputs that drive
the gates of external MOSFETs which, in turn, drive the TEC as
shown in Figure 17. A voltage across the TEC is monitored via
SFB (Pin 23) and LFB (Pin 27). Although both MOSFET drivers
achieve the same result, to provide constant voltage and high
current, their operation is different. The exact equations for the
two outputs are
)25.1(40 −−=
OUT2
BLFB
VVV
)25.1(5 −+=
OUT2
LFB
SFB
VVV
where:
V
OUT2
is the voltage at OUT2 (Pin 7).
V
B
is determined by V
DD
as
]V0.4[V5.1 <=
DDB
VV
]V0.4[V5.2 >=
DDB
VV
The voltage at OUT2 (Pin 7) is determined by the compensation
network that receives temperature set voltage and thermistor
voltage fed by the input amplifier. V
LFB
has a low limit of 0 V
and an upper limit of V
DD
. Figure 21 shows the graphs of these
equations.
0
2.5
5.0
LFB (V)
0
2.5
5.0
SFB (V)
–5.0
–2.5
0
2.5
5.0
0 0.25 0.75 1.25 1.75 2.25 2.75
VTEC (V) LFB-SFB
04663-021
OUT2 (V)
Figure 21. OUT2 Voltage vs. TEC Voltage