LT1619 Linear Technology, LT1619 Datasheet - Page 10

• Manufacturer Part Number: LT1619
• Description: Low Voltage Current Mode PWM Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
(The input impedance of the sense amplifier at the SENSE
pin is 2500 and R
values for R
for R
when this is not desireable.
LT1619
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
V
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 V
S/S pin current is shunted to the ground through the
10
Figure 6. Current Sense Filter for Improving Jitter Performance
COMPARATOR
IN
f
CORNER
exceeds the internal UVLO threshold. With full load,
PWM
OS
reduces Burst Mode threshold; use 10 and 10nF
OS
LT1619
2
and C
R C
1
OS S
OS
U
AMPLIFIER
CURRENT
S
SENSE
is typically less than 137 .) Typical
are 100 and 1nF. The 100 value
IN
exceeds the zener voltage V
U
5
+
–
f
S
5
4
SENSE
GND
W
C
S
R
R
OS
SENSE
U
Z
+
–
V
1619 F06
I
, the
D
SENSE
V
resistor R3. The voltage developed across R3 due to I
should be less than the shutdown threshold. The LT1619
remains off until V
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.
IN
V
R3
Z
Figure 9. Filtering Input Voltage Ripple in UVLO Circuit
I
Figure 7. Implementing Undervoltage Lockout
I
S/S
Figure 8. I-V Characteristics of Zener
and Avalanche Breakdown Diodes
1
2
3
4
V
+
–
IN
S/S
FB
V
GND
C
V
LT1619
C1
0
SENSE
I
GATE
IN
DRV
1619 F07
V
IN
exceeds the sum of V
R4
R3
8
7
6
5
BV < 5V
UVLO THRESHOLD = V
THRESHOLD V
I
I
S/S
S/S
ZENER
DIODE
1
2
3
4
V
V
S/S
S/S
S/S
FB
V
GND
C
= 0
= 0
LT1619
SENSE
R3 < SHUTDOWN THRESHOLD
–2 A
GATE
Z
DRV
1619 F09
+ V
V
IN
AVALANCHE
DIODE
BE
Z
8
7
6
5
V
+ SHUTDOWN
Z
and the
1619fa
S/S