LT1952EGN-1PBF LINER [Linear Technology], LT1952EGN-1PBF Datasheet - Page 13

• Manufacturer Part Number: LT1952EGN-1PBF
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
current (460μA LT1952; 400μA LT1952-1) allows a large
resistor to be connected between system input supply and
V
drive the IC (4.5mA) and the output drivers (I
enough capacitor is chosen at the V
falling below its turn off threshold before an auxiliary winding
in the converter takes over supply to V
allows a simple resistor/capacitor for start-up which draws
low power from the system supply to the converter. The
values for R
Example: (LT1952)
For system input voltages exceeding the absolute maximum
rating of the LT1952/LT1952-1 V
should be connected from the V
covers the condition where V
the part does not turn on because SD_V
condition V
IN
R
C
V
For V
I
I
and t
R
C
(typically choose ≥ 1μF)
. Once the part is turned on, input current increases to
START(MAX)
DRIVE(MAX)
START(MIN)
IN HYST(MIN)
START
START(MAX)
START
D1*
C
START
S(MIN)
START
R
START
= (5.5mA + 5mA) • 100μs/3.75V = 0.28μF
= (36 – 15.75)/700μA = 28.9k (choose 28.7k)
INPUT (V
IN
SYSTEM
START
will continue to charge towards system V
= 100μs,
= 36V, V
= 5mA, V
= 700μA, I
= (I
Figure 4. Low Power Start-Up
= (V
S
)
and C
Q(MAX)
*FOR V
(V
S(MIN)
IN ON(MAX)
V
IN
FROM AUXILIARY WINDING
IN ON(MAX)
S
START
IN HYST(MIN)
> 25V, ZENER D1 RECOMMENDED
Q(MAX)
+ I
(14.25V ON, 8.75V OFF) LT1952
(7.75V ON, 6.5V OFF) LT1952-1
– V
< V
1.32V
DRIVE(MAX)
Z
are given by:
IN ON(max)
IN
< 25V)
= 5.5mA,
charges past V
IN
= 15.75V,
IN
–
+
pin, an external zener
= 3.75V
IN
pin to ground. This
SEC
) • t
pin to prevent V
IN
)/I
. This technique
START(MAX)
< 1.32V. In this
START
DRIVE
1952 F04
IN ON
/
). A large
but
IN
IN
,
possibly exceeding the rating for the V
voltage should obey V
Programming Oscillator Frequency
The oscillator frequency (f
programmed using an external resistor (R
between the R
f
running 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
the 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
LT1952/LT1952-1 to cause premature turn off of SOUT and
OUT in addition to false trigger of soft-start. The LT1952/
LT1952-1 provide programmable leading edge blanking
of the OC and I
current sensing during MOSFET switching.
OSC
R
OSC
vs R
OSC
OSC
= 9.125k [(4100k/f
Figure 5. Oscillator Frequency (f
resistor should be returned directly to the (analog
OSC
node as small as possible. The ground side of
500
450
400
350
300
250
200
150
100
resistor values. The LT1952/LT1952-1 free-
OSC
50
SENSE
pin and ground. Figure 5 shows typical
100
OSC
comparator outputs to avoid false
IN ON(MAX)
150
LT1952/LT1952-1
can be calculated by:
OSC
OSC
200
R
OSC
OSC
) of the LT1952/LT1952-1 is
pin and keeping the area of
SENSE
(kΩ)
) – 1]
250
< V
300
Z
pin thresholds of the
OSC
< 25V.
350
IN
) vs R
OSC
1952 F05
OSC
pin. The zener
400
) connected
OSC
resistor as
13
19521fd
OSC