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参数资料
| 型号： | LT1952MPGN#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 560 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-16 |
| 文件页数： | 6/28页 |
| 文件大小： | 357K |
| 代理商： | LT1952MPGN#PBF |
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LT1952/LT1952-1
14
19521fe
current (460A LT1952; 400A LT1952-1) allows a large
resistor to be connected between system input supply and
VIN. Once the part is turned on, input current increases to
drive the IC (4.5mA) and the output drivers (IDRIVE).Alarge
enough capacitor is chosen at the VIN pin to prevent VIN
fallingbelowitsturnoffthresholdbeforeanauxiliarywinding
in the converter takes over supply to VIN. This technique
allows a simple resistor/capacitor for start-up which draws
low power from the system supply to the converter. The
values for RSTART and CSTART are given by:
RSTART(MAX) = (VS(MIN) – VIN ON(max))/ISTART(MAX)
CSTART(MIN) = (IQ(MAX) + IDRIVE(MAX))tSTART/
VIN HYST(MIN)
Example: (LT1952)
For VS(MIN) = 36V, VIN ON(MAX) = 15.75V,
ISTART(MAX) = 700A, IQ(MAX) = 5.5mA,
IDRIVE(MAX) = 5mA, VIN HYST(MIN) = 3.75V
and tSTART = 100s,
RSTART = (36 – 15.75)/700A = 28.9k (choose 28.7k)
CSTART=(5.5mA+5mA)100s/3.75V=0.28F
(typically choose ≥ 1F)
Forsysteminputvoltagesexceedingtheabsolutemaximum
rating of the LT1952/LT1952-1 VIN pin, an external zener
should be connected from the VIN pin to ground. This
covers the condition where VIN charges past VIN ON but
the part does not turn on because SD_VSEC<1.32V.Inthis
condition VIN will continue to charge towards system VIN,
APPLICATIONS INFORMATION
possibly exceeding the rating for the VIN pin. The zener
voltage should obey VIN ON(MAX) < VZ < 25V.
Programming Oscillator Frequency
The oscillator frequency (fOSC) of the LT1952/LT1952-1 is
programmed using an external resistor (ROSC) connected
between the ROSC pin and ground. Figure 5 shows typical
fOSC vs ROSC 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 ROSC
pin should be minimized by placing the ROSC resistor as
close as possible to the ROSC pin and keeping the area of
the ROSC node as small as possible. The ground side of
theROSCresistorshouldbereturneddirectlytothe(analog
ground) GND pin. ROSC can be calculated by:
ROSC = 9.125k [(4100k/fOSC) – 1]
Figure 4. Low Power Start-Up
1.32V
SYSTEM
INPUT (VS)
FROM AUXILIARY WINDING
*FOR VS > 25V, ZENER D1 RECOMMENDED
(VIN ON(MAX) < VZ < 25V)
1952 F04
VIN
(14.25V ON, 8.75V OFF) LT1952
(7.75V ON, 6.5V OFF) LT1952-1
CSTART
D1*
RSTART
+
–
Figure 5. Oscillator Frequency (fOSC) vs ROSC
ROSC (k)
50
FREQUENCY
(kHz)
400
1952 F05
100 150
250
200
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 ISENSE pin thresholds of the
LT1952/LT1952-1tocauseprematureturnoffofSOUTand
OUT in addition to false trigger of soft-start. The LT1952/
LT1952-1 provide programmable leading edge blanking
of the OC and ISENSE comparator outputs to avoid false
current sensing during MOSFET switching.
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