LT1725IS Linear Technology, LT1725IS Datasheet - Page 16

• Manufacturer Part Number: LT1725IS
• Description: IC CNTRLR ISOLATD FLYBACK 16SOIC
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1725IS

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Voltage - Output: -
• Voltage - Input: -
• Power - Output: -

APPLICATIO S I FOR ATIO
LT1725
amplified. If severe enough, this can cause erratic opera-
tion. For example, assume 3nH of parasitic inductance
(equivalent to about 0.1 inch of wire in free space) is in series
with an ideal 0.025Ω sense resistor. A “zero” will be formed
at f = R/(2πL), or 1.3MHz. Above this frequency the sense
resistor will behave like an inductor.
Several techniques can be used to tame this potential
parasitic inductance problem. First, any resistor used for
current sensing purposes must be of an inherently non-
inductive construction. Mounting this resistor directly
above an unbroken ground plane and minimizing its
ground side connection will serve to absolutely minimize
parasitic inductance. In the case of low valued sense
resistors, these may be implemented as a parallel combi-
nation of several resistors for the thermal considerations
cited above. The parallel combination will help to lower the
parasitic inductance. Finally, it may be necessary to place
a “pole” between the current sense resistor and the
LT1725 I
(see Figure 5). A value of 51Ω is suggested for the resistor,
while the capacitor is selected empirically for the particular
application and layout. Using good high frequency mea-
surement techniques, the I
observed directly with an oscilloscope while the capacitor
value is varied.
SOFT-START FUNCTION
The LT1725 contains an optional soft-start function that is
enabled by connecting an explicit external capacitor be-
tween the SFST pin and ground. Internal circuitry prevents
the control voltage at the V
SFST pin.
16
SGND
PGND
I
SENSE
SENSE
GATE
INDUCTANCE
pin to undo the action of the inductive zero
PARASITIC
C
U
COMP
51Ω
Figure 5
U
C
pin from exceeding that on the
SENSE
R
1725 F05
SENSE
L
P
pin waveform may be
W
SENSE RESISTOR ZERO AT:
COMPENSATING POLE AT:
FOR CANCELLATION:
f =
f =
C
COMP
R
2π(51Ω)C
2πL
SENSE
=
P
R
1
SENSE
COMP
U
L
P
(51Ω)
The soft-start function is enagaged whenever V
is removed, or as a result of either undervoltage lockout
or thermal (overtemperature) shutdown. The SFST node
is then discharged to roughly a V
(Remember that the V
old is deliberately set at a V
millivolts.) When this condition is removed, a nominal
40µA current acts to charge up the SFST node towards
roughly 3V. So, for example, a 0.1µF soft-start capacitor
will place a 0.4V/ms limit on the ramp rate at the V
UVLO PIN FUNCTION
The UVLO pin effects an undervoltage lockout function
with at threshold of roughly 1.25V. An external resistor
divider between the input supply and ground can then be
used to achieve a user-programmable undervoltage lock-
out (see Figure 6a).
An additional feature of this pin is that there is a change in
the input bias current at this pin as a function of the state
of the internal UVLO comparator. As the pin is brought
above the UVLO threshold, the bias current sourced by the
part increases. This positive feedback effects a hysteresis
band for reliable switching action. Note that the size of the
hysteresis is proportional to the Thevenin impedance of
the external UVLO resistor divider network, which makes
it user programmable. As a rough rule of thumb, each 4k
or so of impedance generates about 1% of hysteresis.
(This is based on roughly 1.25V for the threshold and 3µA
for the bias current shift.)
Even in good quality ground plane layouts, it is common
for the switching node (MOSFET drain) to couple to the
UVLO pin with a stray capacitance of several thousandths
of a pF. To ensure proper UVLO action, a 100pF capacitor
is recommended from this pin to ground as shown in
Figure 6b. This will typically reduce the coupled noise to
a few millivolts. The UVLO filter capacitor should not be
made much larger than a few hundred pF, however, as the
hysteresis action will become too slow. In cases where
further filtering is required, e.g., to attenuate high speed
supply ripple, the topology in Figure 6c is recommended.
Resistor R1 has been split into two equal parts. This
provides a node for effecting capacitor filtering of high
C
pin control node switching thresh-
BE
plus several hundred
BE
above ground.
CC
C
power
node.
1725fa