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LTC7862

Rev 0

For more information www.analog.com

OPERATION

High Efficiency Switching Surge Stopper Overview

The LTC7862 is a high efficiency switching surge stop-

per controller. “Normal” operation for the LTC7862 is in

“dropout” mode, in which the top external N-channel

MOSFET is turned on continuously (100% duty cycle)

passing the input voltage through to the output with

minimal voltage drop. During an input overvoltage event,

the LTC7862 switches the gates of the top and bottom

N-channel MOSFETs to act as a switching DC/DC step-

down regulator. This maintains the output voltage at a

safe, user-programmed clamp voltage level. Similarly, the

LTC7862 switches in response to an overcurrent or short-

circuit event. The LTC7862 also switches during startup

to limit inrush current and to smoothly ramp the output

voltage. If the time spent during any of these switching

events exceeds the time programmed by the fault timer

pin (TMR), the LTC7862 will cease switching for a cool-

down period and then attempt to restart.

An LTC7862 high efficiency switching surge stopper can

be thought of as a pre-regulator. Consider as an example

a MIL1275 application, where the input voltage connects

to a nominal 28V vehicle power bus. The power bus can

drop as low as 12V during engine cranking, and can also

surge up to 100V with a total surge duration lasting up to

500ms. A switching surge stopper can be placed between

the input power bus and any downstream load to protect

the load from potentially destructive voltage levels. The

output voltage of the switching surge stopper is pre-regu-

lated or clamped to a safe voltage, such as 34V. Whenever

the input voltage is above 34V, the LTC7862 switches

and limits the output voltage to 34V. Whenever the input

voltage is less than 34V, the LTC7862 is in normal mode

and passes the input voltage through to the output with

no switching.

For both a traditional linear regulator surge stopper (such

as LTC4363) and the LTC7862 high efficiency switching

surge stopper, the power loss increases significantly dur-

ing the input voltage surge when regulation begins. In a

linear regulator surge stopper, the power loss is simply

the output current times the difference between the input

and output voltages, which can be very large. In a switch-

ing surge stopper, the power loss is determined by the

switching regulator’s conversion efficiency, which is

relatively high. This enables the LTC7862 high efficiency

switching surge stopper to deliver much higher output

current and power levels compared to a traditional linear

regulator surge stopper.

Furthermore, by using the LTC7862’s timer, the time spent

switching in regulation can be limited, allowing the oper-

ating power to be pushed beyond what could otherwise

be achieved if the switching regulator were required to

run continuously in steady-state. The use of the timer

improves reliability and reduces component size com-

pared to a continuous solution. The timer also allows

the external components to be optimized for the normal

pass-through mode in which the switching surge stop-

per spends the vast majority of its operating life. In this

normal mode, the switching surge stopper is effectively

acting as a wire with a conduction power loss (sometimes

referred to as an insertion loss) equal to the output cur-

rent times the sum of the resistances of the top MOSFET,

inductor, and current sense resistor.

In particular, a relatively low RDS(ON) (with typically high

gate charge) top MOSFET can be selected. This reduces

power loss in normal operation, but significantly increases

the power loss when switching due to transition losses in

the top MOSFET. Usually, in traditional high input voltage

switching step-down regulators, a low gate charge (with

typically high RDS(ON)) top MOSFET is desired to minimize

transition power losses. But since the LTC7862’s timer

limits the time spent switching, this increased power

loss can be safely tolerated. Likewise, the inductor value

and switching frequency can be chosen to minimize the

insertion loss at the expense of switching losses. This

allows the LTC7862 switching surge stopper to operate at

a higher switching frequency than what would be feasible

in a continuous solution without a timer. This results in a

physically smaller inductor with a lower inductance and

lower DC resistance.

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