LTC3722-2_15 Datasheet, PDF(19/28 Page) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3722-2_15 Datasheet, PDF (19/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

◁

LTC3722-1/LTC3722-2

OPERATION

Current Sensing and Overcurrent Protection

Current sensing provides feedback for the current mode

control loop and protection from overload conditions. The

LTC3722-1/LTC3722-2 are compatible with either resis-

tive sensing or current transformer methods. Internally

connected to the LTC3722-1/LTC3722-2 CS pin are two

comparators that provide pulse-by-pulse and overcurrent

shutdown functions respectively (see Figure 10).

The pulse-by-pulse comparator has a 300mV nominal

threshold. If the 300mV threshold is exceeded, the PWM

cycle is terminated. The overcurrent comparator is set

approximately 2x higher than the pulse-by-pulse level.

If the current signal exceeds this level, the PWM cycle is

terminated, the soft-start capacitor is quickly discharged

and a soft-start cycle is initiated. If the overcurrent condition

persists, the LTC3722-1/LTC3722-2 halts PWM operation

and waits for the soft-start capacitor to charge up to ap-

proximately 4V before a retry is allowed. The soft-start

capacitor is charged by an internal 12ÂµA current source.

If the fault condition has not cleared when soft-start

reaches 4V, the soft-start pin is again discharged and a

new cycle is initiated. This is referred to as hiccup mode

operation. In normal operation and under most abnormal

conditions, the pulse-by-pulse comparator is fast enough

to prevent hiccup mode operation. In severe cases, how-

ever, with high input voltage, very low RDS(ON) MOSFETs

and a shorted output, or with saturating magnetics, the

overcurrent comparator provides a means of protecting

the power converter.

Leading Edge Blanking

The LTC3722-1/LTC3722-2 provides programmable leading

edge blanking to prevent nuisance tripping of the current

sense circuitry. Leading edge blanking relieves the filter-

ing requirements for the CS pin, greatly improving the

response to real overcurrent conditions. It also allows

the use of a ground referenced current sense resistor

or transformer(s), further simplifying the design. With a

single 10k to 100k resistor from RLEB to GND, blanking

times of approximately 40ns to 320ns are programmed. If

not required, connecting RLEB to VREF can disable leading

edge blanking. Keep in mind that the use of leading edge

blanking will set a minimum linear control range for the

phase modulation circuitry.

Resistive Sensing

A resistor connected between input common and the

sources of MB and MD is the simplest method of current

sensing for the full bridge converter. This is the preferred

method for low to moderate power levels. The sense

resistor should be chosen such that the maximum rated

RCS

PULSE BY PULSE

CURRENT LIMIT

BLANK + ÏMOD

300mV â

OVERLOAD

CURRENT LIMIT

650mV â

UVLO

ENABLE

PWM

LATCH

UVLO

PWM ENABLE

LOGIC

H = SHUTDOWN

OUTPUTS

SQ Q

4.1V

0.4V

12ÂµA

CSS

Figure 10. Current Sense/Fault Circuitry Detail

372212 F10

For more information www.linear.com/LTC3722

372212fb

▷