LM5035CSQSLASHNOPB datasheet(15/43 Pages) TI2

15 / 43 page

LM5035C

www.ti.com

SNVS631D – JANUARY 2010 – REVISED OCTOBER 2016

Product Folder Links: LM5035C

Submit Documentation Feedback

Feature Description (continued)

8.3.4 Reference

The REF pin is the output of a 5-V linear regulator that can be used to bias an opto-coupler transistor and

external housekeeping circuits. The regulator output is internally current limited to 15 mA (minimum).

8.3.5 Cycle-by-Cycle Current Limit

The CS pin is driven by a signal representative of the transformer primary current. If the voltage sensed at CS

pin exceeds 0.25 V, the current sense comparator terminates the HO or LO output driver pulse. If the high

current condition persists, the controller operates in a cycle-by-cycle current limit mode with duty cycle

determined by the current sense comparator instead of the PWM comparator. Cycle-by-cycle current limiting may

trigger the hiccup mode restart cycle depending on the configuration of the RES pin (see the following).

A small R-C filter connect to the CS pin and located near the controller is recommended to suppress noise. An

internal 32-Ω MOSFET connected to the CS input discharges the external current sense filter capacitor at the

conclusion of every cycle. The discharge MOSFET remains on for an additional 50 ns after the HO or LO driver

switches high to blank leading edge transients in the current sensing circuit. Discharging the CS pin filter each

cycle and blanking leading edge spikes reduces the filtering requirements and improves the current sense

response time.

The current sense comparator is very fast and responds to short duration noise pulses. Layout considerations

are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be

placed very close to the device and connected directly to the CS and AGND pins. If a current sense transformer

is used, both leads of the transformer secondary should be routed to the filter network, which should be located

close to the IC. If a sense resistor located in the source of the main MOSFET switch is used for current sensing,

a low inductance type of resistor is required. When designing with a current sense resistor, all of the noise

sensitive low power ground connections should be connected together near the AGND pin, and a single

connection should be made to the power ground (sense resistor ground point).

8.3.6 Overload Protection Timer

The LM5035C provides a current limit restart timer to disable the outputs and force a delayed restart (hiccup

mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current limit events required

to trigger the restart is programmable by the external capacitor at the RES pin. During each PWM cycle, the

LM5035C either sources or sinks current from the RES pin capacitor. If no current limit is detected during a

cycle, a 12-µA discharge current sink is enabled to pull the RES pin to ground. If a current limit is detected, the

12-µA sink current is disabled and a 22-µA current source causes the voltage at the RES pin to gradually

increase. The LM5035C protects the converter with cycle-by-cycle current limiting while the voltage at RES pin

increases. If the RES voltage reaches the 2.5-V threshold, the following restart sequence occurs (also see

Figure 11):

•

The RES capacitor and SS capacitors are fully discharged

•

The soft-start current source is reduced from 110 µA to 1 µA

•

The SS capacitor voltage slowly increases. When the SS voltage reaches

≊1 V, the PWM comparator will

produce the first narrow output pulse. After the first pulse occurs, the SS source current reverts to the normal

110-µA level. The SS voltage increases at its normal rate, gradually increasing the duty cycle of the output

drivers

•

If the overload condition persists after restart, cycle-by-cycle current limiting will begin to increase the voltage

on the RES capacitor again, repeating the hiccup mode sequence

•

If the overload condition no longer exists after restart, the RES pin will be held at ground by the 12-µA current

sink and normal operation resumes

The overload timer function is very versatile and can be configured for the following modes of protection:

1. Cycle-by-cycle only: The hiccup mode can be completely disabled by connecting a zero to 50-k

Ω resistor

from the RES pin to AGND. In this configuration, the cycle-by-cycle protection will limit the output current

indefinitely and no hiccup sequences will occur.

2. Hiccup only: The timer can be configured for immediate activation of a hiccup sequence upon detection of

an overload by leaving the RES pin open circuit.

3. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed interval of cycle-by-cycle

limiting before initiating a hiccup mode restart, as previously described. The dual advantages of this

LM5035CSQ/NOPB Datasheet(PDF) 15 Page - Texas Instruments

LM5035CSQ/NOPB Datasheet(HTML) 15 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL