LT1952IGN Linear Technology, LT1952IGN Datasheet - Page 12

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

OPERATIO
APPLICATIO S I FOR ATIO
LT1952/LT1952-1
Slope Compensation
The current mode architecture requires slope compensa-
tion to be added to the current sensing loop to prevent
subharmonic oscillations which can occur for duty cycles
above 50%. Unlike most current mode converters which
have a slope compensation ramp that is fixed internally,
placing a constraint on inductor value and operating
frequency, the LT1952/LT1952-1 have externally adjust-
able slope compensation. Slope compensation can be
programmed by inserting an external resistor (R
series with the I
linear slope compensation ramp which sources current
out of the I
to 35µA at 80% duty cycle.
Over-Current Detection and Soft-Start (OC Pin)
An added feature to the LT1952/LT1952-1 is a precise
100mV sense threshold at the OC pin used to detect
over-current conditions in the converter and set a
Shutdown and Programming Undervoltage Lockout
The LT1952/LT1952-1 have an accurate 1.32V shutdown
threshold at the SD_V
in conjunction with a resistor divider to define the
undervoltage lockout threshold (UVLO) of the system
input voltage (V
current hysteresis (11µA before part turn on, 0µA after
part turn on) allows UVLO hysteresis to be programmed.
Calculation of the ON/OFF thresholds for the supply (SV
to the power converter can be made as follows:
A simple open drain transistor can be added to the resistor
divider network at the SD_V
of the LT1952/LT1952-1 (Figure 3).
The SD_V
be an external source current >11µA to lift the pin past its
1.32V threshold for part turn on.
12
V
V
S OFF
S ON
Threshold = SV
Threshold = 1.32[1 + (R1/R2)]
SEC
SENSE
pin must not be left open since there must
S
SENSE
pin of approximately 8µA at 0% duty cycle
) to the power converter (Figure 3). A pin
U
U
SEC
pin. The LT1952/LT1952-1 have a
IN OFF
pin. This threshold can be used
U
SEC
+ (11µA • R1)
pin to control the turn off
W
U
SLOPE
) in
IN
)
soft-start latch. The OC pin is connected directly to the
source of the primary side MOSFET to monitor peak
current in the MOSFET (Figure 7). The 107mV threshold is
constant over the entire duty cycle range of the converter
because it is unaffected by the slope compensation added
to the I
Synchronizing
A SYNC pin allows the LT1952/LT1952-1 oscillator to be
synchronized to an external clock. The SYNC pin can be
driven from a logic level output, requiring less than 0.8V
for a logic level low and greater than 2.2V for a logic level
high. Duty cycle should run between 10% and 90%. To
avoid loss of slope compensation during synchronization,
the free running oscillator frequency (f
programmed to 80% of the external clock frequency
(f
operation should be increased by 1.25x (= f
Micropower Start-Up: Selection of Start-Up Resistor
and Capacitor for V
The LT1952/LT1952-1 use turn-on voltage hysteresis at
the V
start-up (Figure 4). The LT1952/LT1952-1 monitor V
voltage to allow part turn on at 14.25V (7.75V LT1952-1)
and part turn off at 8.75V (6.5V LT1952-1). Low start-up
SYNC
ON
Figure 3. Programming Undervoltage Lockout (UVLO)
IN
). The R
TRANSISTOR
SHUTDOWN
SENSE
OPTIONAL
pin and low start-up current to allow micro-power
OFF
pin.
SLOPE
INPUT (V
R1
R2
SYSTEM
resistor chosen for non-synchronized
IN
S
)
SD_V
11µA
SEC
1.32V
+
–
LT1952/LT1952-1
OSC
SYNC
) should be
/f
1952 F03
OSC
IN
19521fb
).
pin