Datasheet

LT1952/LT1952-1
14
19521fe
current (460µA LT1952; 400µA LT1952-1) allows a large
resistor to be connected between system input supply and
V
IN
. Once the part is turned on, input current increases to
drive the IC (4.5mA) and the output drivers (I
DRIVE
). A large
enough capacitor is chosen at the V
IN
pin to prevent V
IN
falling below its turn off threshold before an auxiliary winding
in the converter takes over supply to V
IN
. This technique
allows a simple resistor/capacitor for start-up which draws
low power from the system supply to the converter. The
values for R
START
and C
START
are given by:
R
START(MAX)
= (V
S(MIN)
– V
IN ON(max)
)/I
START(MAX)
C
START(MIN)
= (I
Q(MAX)
+ I
DRIVE(MAX)
)•t
START
/
V
IN HYST(MIN)
Example: (LT1952)
For V
S(MIN)
= 36V, V
IN ON(MAX)
= 15.75V,
I
START(MAX)
= 700µA, I
Q(MAX)
= 5.5mA,
I
DRIVE(MAX)
= 5mA, V
IN HYST(MIN)
= 3.75V
and t
START
= 100µs,
R
START
= (36 – 15.75)/700µA = 28.9k (choose 28.7k)
C
START
=(5.5mA+5mA)•100µs/3.75V=0.28µF
(typically choose ≥ 1µF)
For system input voltages exceeding the absolute maximum
rating of the LT1952/LT1952-1 V
IN
pin, an external zener
should be connected from the V
IN
pin to ground. This
covers the condition where V
IN
charges past V
IN ON
but
the part does not turn on because SD_V
SEC
< 1.32V. In this
condition V
IN
will continue to charge towards system V
IN
,
APPLICATIONS INFORMATION
possibly exceeding the rating for the V
IN
pin. The zener
voltage should obey V
IN ON(MAX)
< V
Z
< 25V.
Programming Oscillator Frequency
The oscillator frequency (f
OSC
) of the LT1952/LT1952-1 is
programmed using an external resistor (R
OSC
) connected
between the R
OSC
pin and ground. Figure 5 shows typical
f
OSC
vs R
OSC
resistor values. The LT1952/LT1952-1 free-
running oscillator frequency is programmable in the range
of 100kHz to 500kHz.
Stray capacitance and potential noise pickup on the R
OSC
pin should be minimized by placing the R
OSC
resistor as
close as possible to the R
OSC
pin and keeping the area of
the R
OSC
node as small as possible. The ground side of
the R
OSC
resistor should be returned directly to the (analog
ground) GND pin. R
OSC
can be calculated by:
R
OSC
= 9.125k [(4100k/f
OSC
) – 1]
Figure 4. Low Power Start-Up
1.32V
SYSTEM
INPUT (V
S
)
FROM AUXILIARY WINDING
*FOR V
S
> 25V, ZENER D1 RECOMMENDED
(V
IN ON(MAX)
< V
Z
< 25V)
1952 F04
V
IN
(14.25V ON, 8.75V OFF) LT1952
(7.75V ON, 6.5V OFF) LT1952-1
C
START
D1*
R
START
+
–
Figure 5. Oscillator Frequency (f
OSC
) vs R
OSC
R
OSC
(kΩ)
50
FREQUENCY (kHz)
400
1952 F05
100 150 250200 300 350
500
450
400
350
300
250
200
150
100
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
SENSE
pin thresholds of the
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
SENSE
comparator outputs to avoid false
current sensing during MOSFET switching.