of the PWM clock. The leading edge of OUT_A is delayed

with respect to OUT_B by the overlap time which is deter-

mined by the TIME pin resistor (K1 x RSET) When operating

at maximum duty cycle, the trailing edge of OUT_A corre-

sponds to the trailing edge of the PWM clock. The duty cycle

at OUT_A is therefore always less than the duty cycle of the

clock. The internal clock of the LM5025B operates at a

nominal duty cycle of 75%. If the clock frequency is 400KHz

and the overlap time is set to 100ns, then the maximum

PWM duty cycle will be:

Max Duty Cycle = 75% - 100ns x 400KHz = 71%

Current Limit

The LM5025B contains two modes of over-current protec-

tion. If the sense voltage at the CS1 input exceeds 0.25V the

present power cycle is terminated (cycle-by-cycle current

limit). If the sense voltage at the CS2 input exceeds 0.5V, the

controller will terminate the present cycle, discharge the

softstart capacitor and reduce the softstart current source to

1µA. The softstart (SS) capacitor is released after being fully

discharged and slowly charges with a 1µA current source.

When the voltage at the SS pin reaches approximately 1V,

the PWM comparator will produce the first output pulse at

OUT_A. After the first pulse occurs, the softstart current

source will revert to the normal 20µA level. Fully discharging

and then slowly charging the SS capacitor protects a con-

tinuously over-loaded converter with a low duty cycle hiccup

mode.

These two modes of over-current protection allow the user

great flexibility to configure the system behavior in over-load

conditions. If it is desired for the system to act as a current

source during an over-load, then the CS1 cycle-by-cycle

current limiting should be used. In this case the current

sense signal should be applied to the CS1 input and the CS2

input should be grounded. If during an overload condition it is

desired for the system to briefly shutdown, followed by soft-

start retry, then the CS2 hiccup current limiting mode should

be used. In this case the current sense signal should be

applied to the CS2 input and the CS1 input should be

grounded. This shutdown / soft-start retry will repeat indefi-

nitely while the over-load condition remains. The hiccup

mode will greatly reduce the thermal stresses to the system

during heavy overloads. The cycle-by-cycle mode will have

higher system thermal dissipations during heavy overloads,

but provides the advantage of continuous operation for short

duration overload conditions.

It is possible to utilize both over-current modes concurrently,

whereby momentary overload conditions activate the CS1

cycle-by-cycle mode while prolonged overloading activates

the CS2 hiccup mode. Generally the CS1 input will always

be configured to monitor the main switch FET current each

cycle. The CS2 input can be configured in several different

ways depending upon the system requirements.

a) The CS2 input can also be set to monitor the main switch

FET current except scaled to a higher threshold than CS1

b) An external over-current timer can be configured which

trips after a pre-determined over-current time, driving the

CS2 input high, initiating a hiccup event.

c) In a closed loop voltage regulaton system, the COMP

input will rise to saturation when the cycle-by-cycle current

limit is active. An external filter/delay timer and voltage di-

vider can be configured between the COMP pin and the CS2

pin to scale and delay the COMP voltage. If the CS2 pin

voltage reaches 0.5V a hiccup event will initiate.

A small RC filter, located near the controller, is recom-

mended for each of the CS pins. The CS1 input has an

internal FET which discharges the current sense filter ca-

pacitor at the conclusion of every cycle, to improve dynamic

performance. This same FET remains on an additional 50ns

at the start of each main switch cycle to attenuate the leading

edge spike in the current sense signal. The CS2 discharge

FET only operates following a CS2 event, UVLO and thermal

shutdown.

The LM5025B CS comparators are very fast and may re-

spond 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 pins of the

IC (CS and GND). 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 in the source of the main switch MOSFET 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 IC GND and a single connec-

tion should be made to the power ground (sense resistor

ground point).

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