Rainbow Electronics MAX5971B User Manual
Page 20
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MAX5971B
Single-Port, 40W, IEEE 802.3af/at,
PSE Controller with I
2
C
Foldback Current
During startup and normal operation, an internal circuit
senses the port voltage and reduces the current-limit
value and the overcurrent threshold when (V
AGND
-
V
OUT
) < 27V. The foldback function helps to reduce the
power dissipation on the internal MOSFET. The current
limit eventually reduces down to I
TH_FB
(166mA, typ)
when (V
AGND
- V
OUT
) < 10V (see Figure 3).
Digital Logic
The MAX5971B internally generates digital supplies
(referenced to V
EE
) to power the internal logic circuitry.
All logic inputs and outputs are referenced to V
EE
.
See the Electrical Characteristics table for digital input
thresholds. If digital logic inputs are driven externally, the
nominal digital logic level is 3.3V.
Interrupt
The MAX5971B contains an open-drain logic output
(INT) that goes low when an interrupt condition exists.
The interrupt register (R00h, Table 7) contains the inter-
rupt flag bits and the interrupt mask register (R01h,
Table 8) determines which events can trigger an inter-
rupt. When an event occurs, the appropriate interrupt
event register bits (in R02h through R0Bh) and the cor-
responding interrupt (in R00h) are set to 1 and INT is
asserted low (unless masked).
As a response to an interrupt, the controller can read
the status of the event register(s) to determine the cause
of the interrupt and take appropriate action. Each inter-
rupt event register is paired with a clear-on-read (CoR)
register. When an interrupt event register is read through
the corresponding CoR register, the interrupt register
is reset to 0. INT remains low and the interrupt is not
reset when the interrupt event register is read through
the read-only addresses. For example, to clear a sup-
ply event fault, read R0Bh (CoR) not R0Ah (read only,
see Table 12). Use the CLR_INT bit (R1Ah[7]) to clear
an interrupt, or the RESET_IC (R1Ah[4]) or RESET_P
(R1Ah[0]) bit to initiate a software reset (see Table 27).
Undervoltage and Overvoltage Protection
The MAX5971B contains both undervoltage and over-
voltage protection features. Table 12 in the Register
Map and Description section shows a detailed list of
the undervoltage and overvoltage protection features.
An internal V
EE
undervoltage lockout (V
EE_UVLO
) cir-
cuit keeps the port off and the MAX5971B in reset until
V
AGND
- V
EE
exceeds 28.5V (typ) for more than 2.5ms.
An internal V
EE
overvoltage (V
EE_OV
) circuit shuts down
the port when V
AGND
- V
EE
exceeds 62.5V (typ). The
MAX5971B also features a V
EE
undervoltage interrupt
(V
EE_UV
) that triggers when V
AGND
- V
EE
drops below
Figure 3. Foldback Current Characteristics
V
AGND
- V
OUT
I
LIM
I
TH_FB
10V
27V
I
RSENSE