LT1619 datasheet(10/20 Pages) LINER

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LT1619

1619fa

CORNER

OS S

=≈

(The input impedance of the sense amplifier at the SENSE

pin is 2500

Ω and ROS is typically less than 137Ω.) Typical

values for ROS and CS are 100Ω and 1nF. The 100Ω value

for ROS reduces Burst Mode threshold; use 10Ω and 10nF

when this is not desireable.

Figure 7. Implementing Undervoltage Lockout

–

ZENER

DIODE

AVALANCHE

DIODE

BV < 5V

Figure 8. I-V Characteristics of Zener

and Avalanche Breakdown Diodes

S/S

GND

VIN

DRV

GATE

SENSE

LT1619

VIN

1619 F09

Figure 9. Filtering Input Voltage Ripple in UVLO Circuit

Use of Shutdown Function to

Modify Undervoltage Lockout

The LT1619 is designed to operate from an input supply

with voltage as low as 1.85V. Shutdown is activated when

the S/S pin is pulled below 0.45V. The shutdown threshold

is slightly greater than one junction diode forward voltage

and has the temperature characteristics of a junction

diode. The S/S pin is normally tied to the input when

operating from a low voltage input source.

Consider the 12V to – 65V isolated flyback converter (see

Typical Applications). The converter draws 3A at low line

while delivering 0.4A to the output. If the S/S pin is tied to

the input, then the LT1619 will start switching as soon as

VIN exceeds the internal UVLO threshold. With full load,

the converter can draw much higher than the steady-state

3A from the input source during start-up. If the input

source is current limited, the input voltage will collapse

and latch low.

The start-up problem can be prevented by adding a zener

diode and a resistor to the S/S pin (Figure 7). This is

equivalent to increasing undervoltage lockout voltage of

the controller. Before VIN exceeds the zener voltage VZ, the

S/S pin current is shunted to the ground through the

Figure 6. Current Sense Filter for Improving Jitter Performance

resistor R3. The voltage developed across R3 due to IS/S

should be less than the shutdown threshold. The LT1619

remains off until VIN exceeds the sum of VZ and the

shutdown threshold. True zener diodes (BV < 5V) and

higher voltage avalanche diodes have different I-V charac-

teristics (Figure 8). They need to be biased appropriately

(value of R3) in order to obtain correct UVLO threshold.

When implementing UVLO with converters with high input

ripple voltages (such as flyback and forward), the circuit

in Figure 7 is modified and shown in Figure 9.

PWM

COMPARATOR

CURRENT

SENSE

AMPLIFIER

SENSE

CS RSENSE

VSENSE

–

ROS

GND

1619 F06

LT1619

–

S/S

GND

VIN

DRV

GATE

SENSE

LT1619

VIN

IS/S

1619 F07

IS/S

VS/S = 0

R3 < SHUTDOWN THRESHOLD

≈ –2µA

UVLO THRESHOLD = VZ + SHUTDOWN

THRESHOLD

≈ VZ + VBE

()

IS/S

VS/S = 0

APPLICATIO S I FOR ATIO

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