LT1952IGN Linear Technology, LT1952IGN Datasheet - Page 13

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

APPLICATIO S I FOR ATIO
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
technique 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 maxi-
mum rating of the LT1952/LT1952-1 V
zener should be connected from the V
covers the condition where V
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(MAX)
START
START
D1*
C
START
R
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
will continue to charge towards system V
= 100µs,
= 5mA, V
= 36V, V
= (I
= 700µA, I
Figure 4. Low Power Start-Up
= (V
S
)
*FOR V
(V
Q(MAX)
U
IN ON(MAX)
V
S(MIN)
IN
FROM AUXILIARY WINDING
S
IN ON(MAX)
> 25V, ZENER D1 RECOMMENDED
IN HYST(MIN)
(14.25V ON, 8.75V OFF) LT1952
(7.75V ON, 6.5V OFF) LT1952-1
Q(MAX)
+ I
U
– V
< V
START
1.32V
IN
DRIVE(MAX)
Z
< 25V)
IN ON(max)
charges past V
and C
= 5.5mA,
= 15.75V,
–
+
W
IN
= 3.75V
IN
SEC
START
pin to ground. This
pin to prevent V
) • t
IN
)/I
< 1.32V. In this
pin, an external
START(MAX)
START
DRIVE
are given by:
IN ON
1952 F04
U
/
). A large
IN
but the
. This
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
free-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
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
LT1952-1 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
OSC
R
OSC
OSC
vs. R
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
500
450
400
350
300
250
200
150
100
OSC
50
OSC
SENSE
resistor values. The LT1952/LT1952-1
100
pin and ground. Figure 5 shows typical
OSC
SENSE
150
comparator outputs to avoid false
IN ON(MAX)
LT1952/LT1952-1
can be calculated by,
OSC
OSC
200
R
pin thresholds of the LT1952/
OSC
OSC
) of the LT1952/LT1952-1 is
(kΩ)
pin and keeping the area of
250
) – 1]
< V
300
Z
OSC
< 25V.
350
IN
) vs R
1952 F05
OSC
OSC
pin. The zener
400
) connected
OSC
resistor as
13
19521fb
OSC