LT1952 Linear Technology, LT1952 Datasheet - Page 13

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

APPLICATIO S I FOR ATIO
between system input supply and V
turned on, input current increases to drive the IC (4.5mA)
and the output drivers (I
is chosen at the V
before an auxiliary winding in the converter takes over
supply to V
capacitor for start-up which draws low power from the
system supply to the converter. The values for R
C
Example:
For system input voltages exceeding the absolute maxi-
mum rating of the LT1952 V
be connected from the V
condition where V
but the part does not turn on because SD_V
this condition V
START
R
C
V
For V
I
I
and t
R
C
(typically choose ≥ 1µF)
START(MAX)
DRIVE(MAX)
START(MIN)
IN HYST(MIN)
START(MAX)
START
START
D1*
C
START
R
are given by:
START
S(MIN)
START
INPUT (V
= (5.5mA + 5mA) • 100µs/3.75V = 0.28µF
= (36 – 15.75)/700µA = 28.9k (choose 28.7k)
SYSTEM
IN
. This technique allows a simple resistor/
= 100µs,
= 5mA, V
= 36V, V
= (I
= 700µA, I
Figure 4. Low Power Start-Up
IN
= (V
S
)
IN
IN
will continue to charge towards system
*FOR V
(V
Q(MAX)
V
U
IN ON(MAX)
charges past V
S(MIN)
IN
pin to prevent V
FROM AUXILIARY WINDING
S
IN ON(MAX)
> 25V, ZENER D1 RECOMMENDED
IN HYST(MIN)
DRIVE
IN
Q(MAX)
+ I
U
(14.25V ON, 8.75V OFF)
– V
< V
IN
pin to ground. This covers the
1.32V
DRIVE(MAX)
Z
pin, an external zener should
< 25V)
). A large enough capacitor
IN ON(max)
= 5.5mA,
= 15.75V,
IN ON
–
+
W
IN
= 3.75V
IN
. Once the part is
falling below 8.5V
) • t
(typically 14.25V)
)/I
START(MAX)
START
SEC
LT1952
1952 F04
< 1.32V. In
U
START
/
and
V
voltage should obey V
Programming Oscillator Frequency
The oscillator frequency (f
grammed using an external resistor (R
between the R
f
oscillator frequency is programmable in the range of
100kHz to 500kHz.
Stray capacitance and potential noise pickup on the R
pin should be minimized by placing the R
close as possible to the R
the R
R
ground) GND pin. R
Programming Leading Edge Blank Time
For PWM controllers driving external MOSFETs, noise can
be generated at the source of the MOSFET during gate rise
time and some time thereafter. This noise can potentially
exceed the OC and I
cause premature turn off of SOUT and OUT in addition to
false trigger of soft-start. The LT1952 provides program-
mable leading edge blanking of the OC and I
parator outputs to avoid false current sensing during
MOSFET switching.
OSC
IN
OSC
R
, possibly exceeding the rating for the V
OSC
vs. R
OSC
resistor should be returned directly to the (analog
= 9.125k [(4100k/f
Figure 5. Oscillator Frequency (f
node as small as possible. The ground side of the
OSC
500
450
400
350
300
250
200
150
100
50
OSC
resistor values. The LT1952 free-running
100
pin and ground. Figure 5 shows typical
SENSE
OSC
150
IN ON(MAX)
can be calculated by,
OSC
pin thresholds of the LT1952 to
200
R
OSC
OSC
OSC
(kΩ)
pin and keeping the area of
250
) – 1]
) of the LT1952 is pro-
www.DataSheet4U.com
< V
300
Z
OSC
< 25V.
350
) vs R
1952 F05
OSC
IN
OSC
400
LT1952
pin. The zener
) connected
OSC
SENSE
resistor as
13
com-
OSC
1952f