LT8312

8

8312fa

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operaTion

steady state, but relies on the capacitor to start up the part,

then the auxiliary winding begins to provide power to the

VIN pin along with the resistor. An internal voltage clamp

is attached to the VIN pin to prevent the resistor current

from allowing VIN to go above the absolute maximum

voltage of the pin. The internal clamp is set at 40V and is

capable of 8mA (typical) of current at room temperature.

Setting the VIN Turn-On and Turn-Off Voltages

A large voltage difference between the VIN turn-on voltage

and the VIN turn-off voltage is preferred to allow time for

the auxiliary winding to power the part. The EN/UVLO

sets these two voltages. The pin has a 10μA current sink

when the pins voltage is below 1.25V and 0μA when

above 1.25V. The VIN pin connects to a resistor divider

as shown in Figure 1.

The UVLO threshold for VIN rising is:

VIN(UVLO,RISING) =1.25V •

R1+R2

R2

+10µA •R1

The UVLO Threshold for VIN Falling is :

VIN(UVLO,FALLING) =1.25V •

R1+R2

R2

Programming Output Voltage

The output voltage is set using a resistor divider from the

output capacitor to the FB pin. From the Block Diagram

the resistors R3 and R4 form a resistor divider from the

output capacitor. The output voltage equation is:

VOUT = VBG •

R3+R4

R5

The VBG voltage is equal to FB Voltage in Electrical Speci-

fication Table.

Setting VIN(SENSE) Resistor

TheVIN(SENSE)resistorsetsthecurrentfeedingtheinternal

multiplier that modulates the current limit for power factor

correction.Atthemaximumlinevoltage,VMAX,thecurrent

is set to 360µA. Under this condition, the resistor value is

equal to (VMAX/360µA).

Critical Conduction Mode Operation

Critical conduction mode is a variable frequency switching

scheme that always returns the inductor current to zero

with every cycle. The DCM pin uses a fast current input

comparatorincombinationwithasmallcapacitortodetect

dv/dt on the auxiliary winding. To eliminate false tripping,

a blanking time of 200ns is applied after the switch turns

off. The detector looks for 80μA of current through the

DCM pin due to falling voltage on the auxiliary winding

when the output diode turns off. This is not the optimal

timetoturntheswitchonbecausetheswitchvoltageisstill

close to VOUT and would waste all the energy stored in the

parasitic capacitance on the switch node. Discontinuous

ringing begins when the output diode current reaches zero

EN/UVLO

LT8312

VIN

GND

R2

8312 F01

R1

Figure 1. Undervoltage Lockout (UVLO)