LT1952EGN-1#PBF Linear Technology, LT1952EGN-1#PBF Datasheet - Page 12

• Manufacturer Part Number: LT1952EGN-1#PBF
• Description: IC CTLR PWM SGL SWITCH 16-SSOP
• Manufacturer: Linear Technology

Specifications of LT1952EGN-1#PBF

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 560kHz
• Duty Cycle: 90%
• Voltage - Supply: 6.82 V ~ 25 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 16-SSOP
• Frequency-max: 560kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OPERATION
APPLICATIONS INFORMATION
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 fi xed 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
in series with the I
a linear slope compensation ramp which sources current
out of the I
cycle to 35μA at 80% duty cycle.
Overcurrent 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
overcurrent conditions in the converter and set a soft-start
Shutdown and Programming Undervoltage Lockout
The LT1952/LT1952-1 have an accurate 1.32V shutdown
threshold at the SD_V
used in conjunction with a resistor divider to defi ne the
undervoltage lockout threshold (UVLO) of the system
input voltage (V
current hysteresis (10μA 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 >10μA 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
) to the power converter (Figure 3). A pin
pin of approximately 8μA at 0% duty
SENSE
SEC
IN
pin. The LT1952/LT1952-1 have
OFF + (10μA • R1)
SEC
pin. This threshold can be
pin to control the turn off
SLOPE
IN
)
)
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 synchroniza-
tion, the free running oscillator frequency (f
be 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
SENSE
SYNC
ON
Figure 3. Programming Undervoltage Lockout (UVLO)
IN
). The R
TRANSISTOR
SHUTDOWN
pin.
OPTIONAL
OFF
pin and low start-up current to allow micro-power
SLOPE
INPUT (V
R1
R2
SYSTEM
resistor chosen for non-synchronized
IN
S
)
SD_V
11μA
SEC
1.32V
+
–
LT1952/LT1952-1
SYNC
OSC
) should
1952 F03
/f
OSC
IN
19521fd
).
pin