Datasheet

NTC
THERMISTOR
OVP
LM5045
1.25V
20 PA
STANDBY
V
PWR
5V
R1
R2
T
UVLO
LM5045
1.25V
20 PA
SHUTDOWN
V
PWR
R1
STANDBY
0.4V
STANDBY SHUTDOWN
R2
LM5045
SNVS699G –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 27 shows an application of the OVP comparator for Remote Thermal Protection using a thermistor (or
multiple thermistors) which may be located near the main heat sources of the power converter. The negative
temperature coefficient (NTC) thermistor is nearly logarithmic, and in this example a 100kΩ thermistor with the β
material constant of 4500 Kelvin changes to approximately 2kΩ at 130ºC. Setting R
1
to one-third of this
resistance (665Ω) establishes 130ºC as the desired trip point (for VREF = 5V). In a temperature band from 20ºC
below to 20ºC above the OVP threshold, the voltage divider is nearly linear with 25mV per ºC sensitivity.
R
2
provides temperature hysteresis by raising the OVP comparator input by R
2
x 20µA. For example, if a 22kΩ
resistor is selected for R
2
, then the OVP pin voltage will increase by 22k x 20µA = 506mV. The NTC temperature
must therefore fall by 506mV / 25mV per ºC = 20ºC before the LM5045 switches from standby mode to the
normal mode.
Figure 26. Remote Standby and Disable Control
Figure 27. Remote Thermal Protection
Current Sense
The CS pin receives an input signal representative of its transformer’s primary current, either from a current
sense transformer or from a resistor located at the junction of source pin of the primary switches, as shown in
Figure 28 and Figure 29 respectively. In both the cases, the filter components R
F
and C
F
should be located as
close to the IC as possible, and the ground connection from the current sense transformer, or R
SENSE
should be
a dedicated trace to the appropriate GND pin. Please refer to the Printed Circuit Board Layout section for more
layout tips.
The current sense components must provide a signal > 710mV at the CS pin during an over-load event. Once
the voltage on the CS pin crosses the current limit threshold, the current sense comparator terminates the PWM
pulse and starts to charge the RES pin. Depending on the configuration of the RES pin, the LM5045 will
eventually initiate a hiccup mode restart or be in continuous current limit.
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