lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 22

• Manufacturer Part Number: lm5035mhx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Information
then leakage current in the zener diode amplified by the
opto-coupler’s gain could falsely trip the protection latch. R1
and Q1 are added reduce the sensitivity to low level currents
in the opto-coupler. Using the values of Figure 14, the opto-
Figure 15 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 supply. The negative temperature co-
efficient (NTC) thermistor is nearly logarithmic, and in this
example a 100kΩ thermistor with the β material constant of
4500 kelvins changes to approximately 2kΩ at 130˚C. Set-
ting R1 to one-third of this resistance (665Ω) establishes
130˚C as the desired trip point (for V
ture band from 20˚C below to 20˚C above the OVP thresh-
old, the voltage divider is nearly linear with 25mV per˚C
sensitivity.
HICCUP MODE CURRENT LIMIT RESTART (RES)
The basic operation of the hiccup mode current limit restart
is described in the functional description. The delay time to
restart is programmed with the selection of the RES pin
capacitor C
RES
as illustrated in Figure 15.
REF
FIGURE 14. Latched Load Over-Voltage Protection
= 5V). In a tempera-
FIGURE 15. Remote Thermal Protection
(Continued)
22
coupler collector current must equal V
before OVP latches. Once the controller has switched to
standby mode, the outputs no longer switch but the VCC and
REF regulators continue functioning and supply bias to the
external circuitry. VCC must fall below 6.2V or the UVLO pin
must fall below 0.4V to clear the OVP latch.
R2 provides temperature hysteresis by raising the OVP com-
parator input by R2 x 23µA. For example, if a 22kΩ resistor
is selected for R2, then the OVP pin voltage will increase by
22kΩ x 23µA = 506mV. The NTC temperature must therefore
fall by 506mV / 25mV per˚C = 20˚C before the LM5035
switches from the standby mode to the normal mode.
20177538
20177537
BE(Q1)
/ R1 = 350µA