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
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 performs true ‘leading edge blanking’ by auto-
matically blanking OC and I
OUT rises to within 0.5V of V
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 uses a precise 100mV sense threshold 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 compensation pro-
grammed at the I
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
SENSE
U
pin. The OC pin monitors the peak
10k < R
(PROGRAMMABLE)
S
U
SENSE
) in the source of the MOSFET.
EXTENDED
BLANKING
IN
BLANK
or reaches its clamp level
BLANK
comparator outputs until
≤ 240k
W
/10k)]ns
[X + 45(R
BLANK
CURRENT
BLANK
SENSE
DELAY
100ns
/10k)]ns
U
= 10k to
1952 F06
The current limit for the converter can be programmed by,
where,
Programming Slope Compensation
The LT1952 uses a current mode architecture to provide
fast response to load transients and to ease frequency
compensation requirements. Current mode switching regu-
lators which operate with duty cycles above 50% and have
continuous inductor current must add slope compensa-
tion to their current sensing loop to prevent subharmonic
oscillations. (For more information on slope compensa-
tion, see Application Note 19.) The LT1952 has program-
mable slope compensation to allow a wide range of
inductor values, to reduce susceptibility to PCB generated
noise and to optimize loop bandwidth. The LT1952 pro-
grams slope compensation by inserting a resistor R
in series with the I
generates a current at the I
0% duty cycle to the maximum duty cycle of the OUT pin.
A simple calculation of I(I
ramp to the voltage at the I
slope compensation. (See both graphs ‘I
vs. Duty Cycle’ and ‘I
Cycle’ in the Typical Performance Characteristics
section.)
Current limit = (100mV/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
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
R
SLOPE
SENSE
SENSE
SENSE
V
R
S
SENSE
S
pin (Figure 7). The LT1952
Maximum Threshold vs Duty
SENSE
S
)(N
V
I
DC = DUTY CYCLE
FOR SYNC OPERATION
I
k = f
SENSE
SENSE(SYNC)
(ISENSE)
) • R
OSC
P
pin which is linear from
/N
= 8µA + 35DC µA
pin for programmable
/f
SLOPE
SYNC
= V
S
) – (1/2)(I
S
= 8µA + (k • 35DC)µA
+ (I
SENSE
SENSE
gives an added
• R
Pin Current
RIPPLE
SLOPE
)
SLOPE
)
1952f