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LTC4366

436612fe

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APPLICATIONS INFORMATION

VDD

470k

100k

OUT

2nF

GATE

IXTH12N100L

3.3nF

0.47µF

RFB1

12.4k

VOUT

0.5A

(200V CLAMP)

VIN

160V (RECTIFIED 110V AC)

100V TO 800V

RFB2

436612 F07

RSS

412k

RIN

4.64M

BF722

LTC4366-2

TIMER

BASE

VSS

DANGER! Lethal Voltages Present

Figure 7. Rectified 110V AC Supply Protected from 220V AC

times time or P2t. Knowing the power we then adjust

the time using the timer capacitor to limit the P2t during

overvoltage. In this example the MOSFET data sheet has

a 6400W2s P2t for a 10ms single pulse.

In this application 250V minus 43V is applied across the

MOSFET at 3A. If the power is applied for less than 16.5ms

then MOSFET P2t limit is not exceeded:

P = (250V – 43V) • 3A = 621W

P2t = (621W)2 • 16.5ms = 6363W2s

Prior to the moment when the output is regulated at 43V,

the output is ramping from 28V to 43V. This ramp time is

based on the 20µA gate current charging the 10nF capaci-

tor. Using the equation for ramp time:

∆t = CG •

∆V

= 10nF • 15V

20µA

= 7.5ms

To be safe we set the overvoltage time to 10ms. We set the

regulation time to be 2.5ms (the remainder of the 10ms

overvoltage time minus the ramp time). In this example

it is assumed the 250V overvoltage is a constant DC volt-

age for 10ms. This duration exceeds Mil-Std-1275 which

specifies a 70µs surge to 250V that decays in 1.6ms. Us-

ing the following equation (based on charging with 9µA)

to set the CT:

CT =IT •

∆t

∆V

= 9µA •

2.5ms

2.8V

≈ 8.2nF

In order to limit the SD pin current (10mA max) a collector

resistor, R1, in series with Q1 is required. The maximum

value for this resistor is around 5M. This requirement oc-

curs when the pull-down is required to sink 1.6µA from

SD and VDD is clamped at 12V. High valued resistors are

susceptibletoleakagecurrentssowechosea470kresistor

for R1. Resistor R2 provides ESD protection for Q1’s base.

The gate resistor RG limits the parasitic trace capacitance

on M1’s gate node that could lead to parasitic MOSFET

self-oscillation. The recommended value for RG is 10Ω.

High Voltage Application

In Figure 7 the circuit accepts 110V AC (rectified to 160V)

and protects the load from accidental connection to 220V

AC by limiting the output to less than 200V. The circuit

has a 100V to 800V VIN operating range where the FET

breakdown voltage limits the maximum input voltage. The

C1issetto0.47µFtoprovideabypassforthechargepump

that is large enough to provide good noise immunity from

outside voltage transients. The timer capacitor is sized to

give a 1ms overvoltage regulation time that keeps the P2t

below the 640W2s specified for this MOSFET.

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