LT1248IS Linear Technology, LT1248IS Datasheet - Page 7

• Manufacturer Part Number: LT1248IS
• Description: IC PFC CTRLR AVERAGE CURR 16SOIC
• Manufacturer: Linear Technology

Specifications of LT1248IS

• Mode: Average Current
• Frequency - Switching: 300kHz
• Current - Startup: 250µA
• Voltage - Supply: 15.5 V ~ 27 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

PI FU CTIO S
Pin 12 (R
oscillator charging current and the maximum multiplier
output current which is used to limit the maximum line
current.
Pin 13 (SS): Soft-Start. When either V
low, the SS pin will stay at 0V. With a capacitor from the
pin to GND, the 12 A charging current slowly brings up the
SS to 8V; below 7.5V SS is the reference input to the
voltage amplifier. At supply dropout or EN/SYNC low, the
soft start capacitor will be quickly discharged.
Pin 14 (C
R
is 5V, and the frequency = 1.5/(R
Error Amplifier
The error amplifier has a 100dB DC gain and 3MHz unity-
gain frequency. The output is internally clamped at 13.5V.
The noninverting input is tied to the 7.5V V
diode and can be pulled down from the SS (soft-start) pin.
Current Amplifier
The current amplifier has a 110dB DC gain, 3MHz unity-
gain frequency, and a 2V/ s slew rate. It is internally
clamped at 8.5V. Note that in the current averaging opera-
tion, high gain at twice the line frequency is necessary to
minimize line current distortion. Because CA
to swing 5V over one line cycle at high line condition,
14mV AC will be needed at the inputs of the current
amplifier for a gain of 350 at 120Hz. Especially at light load
when the current loop reference signal is small, lower gain
will distort the reference signal and line current. If signal
gain at switching frequency is too high, the system be-
haves more like a current mode system and can cause
subharmonic oscillation. Therefore, the current amplifier
should be compensated to have a gain of less than 15 at
the switching frequency, but more than 250 at twice the
line frequency.
A
SET
I
PPLICATI
U
M(MAX)
, determine oscillator frequency. The oscillator ramp
SET
SET
U
= 3.75V/R
): The capacitor from this pin to GND, and
): A resistor from R
O
U
SET
U
S
I FOR ATIO
U
SET
SET
W
CC
• C
to GND sets the
or EN/SYNC goes
SET
).
REF
OUT
through a
may need
U
Pin 15 (V
has a very fast gate driver required to fast charge high
power MOSFET gate capacitance. High current spikes
occur during charging. For good supply bypass, a 0.1 F
ceramic capacitor in parallel with a low ESR electrolytic
capacitor, 56 F or higher is required in close proximity to
IC GND.
Pin 16 (GTDR): The MOSFET gate driver is a 1.5A fast
totem pole output. It is clamped at 15V, but capacitive
loads like MOSFET gates may cause overshoot. A gate
series resistor of at least 5 will prevent the overshoot.
Multiplier
The multiplier is a current multiplier with high noise
immunity in a high power switching environment. The
current gain is: I
– 2V)/25k. With a square function, because of the lower
gain at light power load, system stability is maintained and
line current distortion caused by the line frequency AC
Figure 1. Multiplier Current I
CC
300
150
): This is the supply for the chip. The LT1248
0
0
M
VA
VA
= (I
OUT
OUT
VA
AC
= 6V
= 6.5V
OUT
• I
= 7V
EA
I
AC
250
2
( A)
)/(200 A)
M
vs I
AC
2
, with I
1248 G04
and VA
500
VA
VA
VA
VA
VA
VA
VA
LT1248
OUT
OUT
OUT
OUT
OUT
OUT
OUT
EA
OUT
= (VA
= 5.5V
= 5V
= 4.5V
= 4V
= 3.5V
= 3V
= 2.5V
7
OUT