LT1952 Linear Technology, LT1952 Datasheet - Page 12

• Manufacturer Part Number: LT1952
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: Linear Technology

OPERATIO
APPLICATIO S I FOR ATIO
LT1952
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 has externally adjustable slope
compensation. Slope compensation can be programmed
by inserting an external resistor (R
I
ramp which sources current out of the I
approximately 8µA at 0% duty cycle to 35µA at 80% duty
cycle.
Over-Current Detection and Soft-Start (OC Pin)
An added feature to the LT1952 is a precise 100mV sense
threshold at the OC pin used to detect over-current condi-
tions in the converter and set a soft-start latch. The OC pin
Shutdown and Programming Undervoltage Lockout
The LT1952 has an accurate 1.32V shutdown threshold at
the SD_V
tion with a resistor divider to define the undervoltage
lockout threshold (UVLO) of the system input voltage (V
to the power converter (Figure 3). A pin 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
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 (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
SENSE
V
V
S OFF
S ON
pin. The LT1952 has a linear slope compensation
Threshold = SV
SEC
Threshold = 1.32[1 + (R1/R2)]
SEC
pin. This threshold can be used in conjunc-
pin must not be left open since there must
U
U
IN OFF
U
SEC
+ (11µA • R1)
pin to control the turn off
SLOPE
W
) in series with the
IN
) to the power
SENSE
U
pin of
S
)
is connected directly to the source of the primary side
MOSFET to monitor peak current in the MOSFET
(Figure 7). The 100mV 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 oscillator to be synchro-
nized 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
grammed to 80% of the external clock frequency (f
The R
tion should be increased by 1.25x (= f
Micropower Start-Up: Selection of Start-Up Resistor
and Capacitor for V
The LT1952 uses turn-on voltage hysteresis at the V
and low start-up current to allow micro-power start-up
(Figure 4). The LT1952 monitors V
part turn on at 14.25V and part turn off at 8.75V. Low start-
up current (460µA) allows a large resistor to be connected
SENSE
ON
Figure 3. Programming Undervoltage Lockout (UVLO)
SLOPE
TRANSISTOR
SHUTDOWN
pin.
OPTIONAL
OFF
resistor chosen for non-synchronized opera-
INPUT (V
R1
R2
SYSTEM
IN
S
)
SD_V
11µA
SEC
1.32V
www.DataSheet4U.com
IN
OSC
pin voltage to allow
+
–
SYNC
) should be pro-
/f
OSC
LT1952
).
1952 F03
SYNC
IN
1952f
pin
).