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PDF LT1508C Data sheet ( Hoja de datos )
| Número de pieza | LT1508C | |
| Descripción | Power Factor and PWM Controller(Voltage Mode) | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s PFC and PWM Single Chip Solution
s Synchronized Operation up to 300kHz
s 99% Power Factor Over 20:1 Load Current Range
s Voltage Mode PWM
s Instantaneous Overvoltage Protection
s Dedicated Overvoltage Protection (OVP Pin)
s Minimal Line Current Dead Zone
s Typical 250µA Start-Up Supply Current
s Line Switching Noise Filter
s Low Quiescent Current: 13mA
s Fast 1.5A Peak Current Gate Drivers
s Separate Soft Start Control
U
APPLICATIONS
s Universal Power Factor Corrected Power Supplies
and Preregulators
LT1508
Power Factor and PWM
Controller (Voltage Mode)
DESCRIPTION
The LT ®1508 is a complete solution for universal off-line
switching power supplies utilizing active power factor
correction. The PFC section is identical to the LT1248 PFC
controller except the EN/SYNC pin is removed because
PFC and PWM are synchronized internally.
The voltage mode PWM section (LT1509 is the current
mode counterpart) contains all the primary side functions
to convert the PFC preregulated high voltage output to an
isolated low voltage output. The PWM duty cycle is
internally limited to 47% (maximum 50%) to prevent
transformer saturation. PWM soft start begins when the
PFC output reaches the preset voltage. In the event of brief
line loss, PWM will be shut off when the PFC output
voltage drops below 73% of the preset value.
, LTC and LT are registered trademarks of Linear Technology Corporation.
BLOCK DIAGRAM
VCC
16V TO 10V
VSENSE
14
7.5V
IAC
9
7.9V
OVP
11
SS1 14µA
16
14µA
SS2
13
+
–
–
EA
+
+
–
VAOUT
10
7.5V
VREF
VREF
12
MOUT ISENSE
87
RUN
2.2V +
M1
–
IA
25k
IB
IM =
IA2IB
200µA2
7µA
IM
–
CA
+
R
2R
–
+
PWMOK
+
1V
–
7V TO
4.7V
–
CL
+
+
–
NOTE: PWM PULSE IS DELAYED BY 55% DUTY CYCLE
AFTER PFC PULSE
19 ILIM
VC 18 50µA
CAOUT
6
GND1
3
–
+
+
0.7V
–
R
R
RUN S
Q
OSC
4 CSET
RSET 15
55%
DELAY
S
RQ
R
200ns
BLANKING
PKLIM VCC
5 17
–
+
GTDR1
1
16V
GND2
2
GTDR2
20
16V
1508 BD
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LT1508
Reference Voltage
vs Temperature
7.536
7.524
7.512
7.500
7.488
7.476
7.464
7.452
7.440
7.428
–75 –50 –25 0 25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
1508 G04
Multiplier Current
300
VAOUT = 7V
VAOUT = 6.5V
VAOUT = 6V
150
0
0 250
IAC (µA)
VAOUT = 5.5V
VAOUT = 5V
VAOUT = 4.5V
VAOUT = 4V
VAOUT = 3.5V
VAOUT = 3V
VAOUT = 2.5V
500
1508 G05
Supply Current vs Supply Voltage
16
15
14 TJ = –55°C
13
12 TJ = 125°C TJ = 25°C
11
10
9
8
7
6
5
10 21 32
SUPPLY VOLTAGE (V)
1508 G06
Frequency vs RSET and CSET
500
450 RSET = 10k
RSET = 15k
400 RSET = 20k
350 RSET = 30k
300
250
200
150
100
50
0
200 600 1000 1400 1800 2200
CSET CAPACITANCE (pF)
1508 G09
GTDR Rise and Fall Time
400
300
200
100
0
0
FALL TIME
RISE TIME
NOTE: GTDR SLEWS
BETWEEN 1V AND 16V
10 20 30 40
LOAD CAPACITANCE (nF)
50
1508 G07
GTDR1 Maximum Duty Cycle
vs RSET and CSET
1.00
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
0.91
0.90
200
RSET = 10k
RSET = 15k
RSET = 20k
RSET = 30k
600 1000 1400 1800 2200
CSET CAPACITANCE (pF)
1508 G10
Start-Up Supply Current
vs Supply Voltage
550
500
450
400
350
300 –55°C
250 25°C
200 125°C
150
100
50
0
02
4 6 8 10 12 14 16 18 20
SUPPLY VOLTAGE (V)
1508 G08
MOUT Pin Characteristics
1.5
1.0 TJ = 125°C
TJ = 25°C
0.5 TJ = –55°C
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–2.4
–1.2 0
1.2
MOUT VOLTAGE (V)
2.4
1508 G11
5
5 Page 
LT1508
APPLICATIONS INFORMATION
input up to VC and then the SS2 voltage continues beyond
VC. The PWMOK comparator contains hysteresis and will
pull SS2 low disabling the PWM section if the PFC output
voltage falls below approximately 62% of its preset value
(240V with nominal 382V output).
Start-Up and Supply Voltage
The LT1508 draws only 250µA before the chip starts at
16V on VCC. To trickle start, a 91k resistor from the power
line to VCC supplies trickle current, and C4 holds VCC up
while switching starts (see Figure 8); then the auxiliary
winding takes over and supplies the operating current.
Note that D3 and the larger values of C3 are only necessary
for systems that have sudden large load variations down
to minimum load and/or very light load conditions. Under
these conditions the loop may exhibit a start/restart mode
because switching remains off long enough for C4 to
discharge below 10V. Large values for C3 will hold VCC up
until switching resumes. For less severe load variations D3
is replaced with a short and C3 is omitted. The turns ratio
between the primary winding determines VCC
according to :
VOUT = NP
VCC – 2V NS
for 382V VOUT and 18V VCC, Np/Ns ≈ 19.
LINE MAIN INDUCTOR
NP
NS
R1
91k
1W
D1
D2
D3
+ C1
VCC
2µF + C3 + C4
+ C2
2µF
390µF
100µF
Figure 8
1508 • F08
Output Capacitor (PFC Section)
GTDR2 (PWM) pulse is synchronized to GTDR1 (PFC) pulse
with 53% duty cycle delay to reduce RMS ripple current in the
output capacitor. See PFC/PWM Synchronization graph in
the Typical Performance Characteristics section.
The peak-to-peak 120Hz PFC output ripple is determined by:
VP-P = 2ILOAD(DC)(Z)
where ILOAD(DC) is the DC load current of the PWM stage
and Z is the capacitor impedance at 120Hz.
For 470µF, impedance is 2.8Ω at 120Hz. At 335W load,
ILOAD(DC) = 335V/382V = 0.88A, VP-P = (2)(0.88)(2.8Ω) =
5V. If less ripple is desired higher capacitance should be
used. The selection of the output capacitor is based on
voltage ripple, hold-up time and ripple current. Assuming
the DC converter (PWM section) is designed to operate
with 240V to 382VIN , the minimum hold-up time is a
function of the energy storage capacity of the capacitor:
tHOLD =
(0.5)COUT
POUT
(382V – 0.5VP–P)2 – 240V2
with COUT = 470µF, VP-P = 11.5V, and POUT = 335W,
tHOLD = 60ms which is 3.6 line cycles at 60Hz. The ripple
current can be divided into two major components. The
first is the 120Hz component which is related to the DC
load current as follows:
I120HZ ≈ ILOAD(DC) √2
The second component is made up of switching frequency
components due to the PFC stage charging the capacitor
and the PWM stage discharging the capacitor. For a 300W
output PFC forward converter running from an input
voltage of 100VRMS, the total high frequency ripple current
was measured to be 1.79ARMS.
For the United Chemicon KMH 450V capacitor series,
ripple current at 100kHz is specified 1.43 times higher
than the 120Hz limit.
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1508C.PDF ] | |
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