Datasheet

7
LT1248
PI FU CTIO S
U
UU
Pin 12 (R
SET
): A resistor from R
SET
to GND sets the
oscillator charging current and the maximum multiplier
output current which is used to limit the maximum line
current.
I
M(MAX)
= 3.75V/R
SET
Pin 13 (SS): Soft start. When either V
CC
or EN/SYNC goes
low, the SS pin will stay at 0V. With a capacitor from the
pin to GND, the 12µA 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
SET
): The capacitor from this pin to GND, and
R
SET
, determine oscillator frequency. The oscillator ramp
is 5V, and the frequency = 1.5/(R
SET
× C
SET
).
Pin 15 (V
CC
): This is the supply for the chip. The LT1248
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 0.1µF
ceramic capacitor in parallel with a low ESR electrolytic
capacitor, 56µF 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 5Ω will prevent the overshoot.
I
AC
(µA)
0
I
M
(µA)
300
150
0
1248 G04
250
500
VA
OUT
= 7V
VA
OUT
= 6.5V
VA
OUT
= 6V
VA
OUT
= 5.5V
VA
OUT
= 5V
VA
OUT
= 4.5V
VA
OUT
= 4V
VA
OUT
= 3.5V
VA
OUT
= 3V
VA
OUT
= 2.5V
Figure 1. Multiplier Current I
M
vs I
AC
and VA
OUT
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Multiplier
The multiplier is a current multiplier with high noise immu-
nity in a high power switching environment. The current
gain is: I
M
= (I
AC
× I
EA
2
)/(200µA)
2
, with I
EA
= (VA
OUT
– 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 ripple
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
REF
through a
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/µs 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
OUT
may need
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.