LT1952IGN Linear Technology, LT1952IGN Datasheet - Page 14

• Manufacturer Part Number: LT1952IGN
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,SOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1952/LT1952-1
Blanking is provided in 2 phases (Figure 6): The first phase
automatically blanks during gate rise time. Gate rise times
can vary depending on MOSFET type. For this reason the
LT1952/LT1952-1 perform true ‘leading edge blanking’ by
automatically blanking OC and I
until OUT rises to within 0.5V of V
level of 13V. The second phase of blanking starts after the
leading edge of OUT has been completed. This phase is
programmable by the user with a resistor connected from
the BLANK pin to ground. Typical durations for this portion
of the blanking period are from 45ns at R
540ns at R
mated as:
(see graph in Typical Performance Characteristics)
Programming Current Limit (OC Pin)
The LT1952/LT1952-1 use a precise 107mV sense thresh-
old at the OC pin to detect over-current conditions in the
converter and set a soft-start latch. It is independent of
duty cycle because it is not affected by slope compensa-
tion programmed at the I
the peak current in the primary MOSFET by sensing the
voltage across a sense resistor (R
14
OUT
BLANKING
Blanking (extended) = [45(R
(AUTOMATIC)
BLANKING
LEADING
0
BLANK
EDGE
Figure 6. Leading Edge Blank Timing
Xns X + 45ns
R
= 10k
= 120k. Blanking duration can be approxi-
(MIN)
BLANK
U
10k < R
SENSE
(PROGRAMMABLE)
U
EXTENDED
BLANKING
BLANK
SENSE
BLANK
pin. The OC pin monitors
IN
≤ 240k
S
W
) in the source of the
or reaches its clamp
comparator outputs
/10k)]ns
[X + 45(R
BLANK
CURRENT
BLANK
SENSE
DELAY
100ns
/10k)]ns
U
= 10k to
1952 F06
MOSFET. The current limit for the converter can be
programmed by,
where,
Programming Slope Compensation
The LT1952/LT1952-1 use a current mode architecture to
provide fast response to load transients and to ease
frequency compensation requirements. Current mode
switching regulators which operate with duty cycles above
50% and have continuous inductor current must add
slope compensation to their current sensing loop to
prevent subharmonic oscillations. (For more information
on slope compensation, see Application Note 19.) The
LT1952/LT1952-1 have programmable slope compensa-
tion to allow a wide range of inductor values, to reduce
susceptibility to PCB generated noise and to optimize loop
bandwidth. The LT1952/LT1952-1 program slope com-
pensation by inserting a resistor R
I
current at the I
to the maximum duty cycle of the OUT pin. A simple
calculation of I(I
the voltage at the I
compensation. (See both graphs ‘I
Duty Cycle’ and ‘I
Cycle’ in the Typical Performance Characteristics section.)
SENSE
Current limit = (107mV/R
R
I
N
N
RIPPLE
S
S
P
= sense resistor in source of primary MOSFET
= number of transformer secondary turns
= number of transformer primary turns
pin (Figure 7). The LT1952/LT1952-1 generate a
LT1952-1
LT1952/
CURRENT SLOPE = 35µA • DC
Figure 7. Programming Slope Compensation
= p-p ripple current in the output inductor L1
I
SENSE
OUT
1952 F07
OC
SENSE
SENSE
R
SENSE
SLOPE
pin which is linear from 0% duty cycle
SENSE
) • R
Maximum Threshold vs Duty
V
R
SLOPE
S
S
pin for programmable slope
S
)(N
V
I
DC = DUTY CYCLE
FOR SYNC OPERATION
I
k = f
SENSE
SENSE(SYNC)
(ISENSE)
OSC
P
gives an added ramp to
/N
SLOPE
= 8µA + 35DC µA
/f
SENSE
SYNC
= V
S
) – (1/2)(I
S
= 8µA + (k • 35DC)µA
+ (I
in series with the
SENSE
Pin Current vs.
• R
RIPPLE
SLOPE
)
19521fb
)