Rainbow Electronics MAX5940B - Manual (Page 8)

MAX5940A/MAX5940B
To adjust the UVLO threshold (MAX5940B only), con-
nect an external resistor-divider from GND to UVLO
and from UVLO to V
EE . Use the following equations to
calculate R1 and R2 for a desired UVLO threshold:
R1 = 25.5k Ω - R2
where V IN,EX is the desired UVLO threshold. Since the
resistor-divider replaces the 25.5k Ω PD detection resis-
tor, ensure that the sum of R1 and R2 equals 25.5k Ω
±1%. When using the external resistor-divider, the
MAX5940B has an external reference voltage hystere-
sis of 20% (typ). When UVLO is programmed external-
ly, the turn-off threshold is 80% (typ) of the new UVLO
threshold.
R k x
V
V
REF UVLO
IN EX
2 25 5 = .
,
,
Ω
IEEE 802.3af PD Interface Controller
For Power-Over-Ethernet
8 _______________________________________________________________________________________
R1
GND
UVLO
GND
(UVLO)
( ) MAX5940B.
GATE
R2
R3
MAX5940B
CLASSIFICATION RCLASS
(PGOOD)
6.8V EN
REF
2.46V
200mV
V EE
V GATE
1.2V, REF
5V, REF
Q4
PGOOD
OUT
Q3
Q1
Q2
EN
20%
Figure 2. Block Diagram
R1
UVLO
GND
V EE
R2
V IN = 12V TO 67V
MAX5940B
Figure 3. Setting Undervoltage Lockout with an External
Resistor-Divider