LX1552 Microsemi Corporation, LX1552 Datasheet - Page 15

• Manufacturer Part Number: LX1552
• Description: ULTRA-LOW START-UP CURRENT/ CURRENT-MODE PWM
• Manufacturer: Microsemi Corporation

Copyright © 1994
Rev. 1.0a 1/01
CURRENT SENSE COMPARATOR AND PWM LATCH
Switch current is sensed by an external sense resistor (or a current
transformer), monitored by the C.S. pin and compared internally
with voltage from error amplifier output. The comparator output
resets the PWM latch ensuring that a single pulse appears at the
output for any given oscillator cycle. The LX1554/55 series has
an additional flip flop stage that limits the output to less than 50%
duty cycle range as well as dividing its output frequency to half
of the oscillator frequency.
threshold is internally clamped to 1V nominally which would
limit peak switch current to:
Equation 1 is used to calculate the value of sense resistor during
the current limit condition where switch current reaches its
maximum level.
relationship between peak switch current and error voltage
(voltage at pin 1) is given by:
The above equation is plotted in Figure 29. Notice that the gain
becomes non-linear above current sense voltages greater than
0.95 volts. It is therefore recommended to operate below this
range during normal operation. This would insure that the overall
closed loop gain of the system will not be affected by the change
in the gain of the current sense stage.
FIGURE 29 — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
(1) I
(1) I
SP
1.1
1.0
0.9
0.8
0.1
=
0.7
0.6
0.5
0.4
0.3
0.2
SP
0
=
V
0
E
3 * R
- 2V
V
R
0.5
Z
S
Error Amplifier Output Voltage - (V)
S
F
In normal operation of the converter, the
1.0
where:
1.5
where:
T
A
2.0
= 25°C
2.5
The current sense comparator
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
T
I
V
A
V
V
SP
Z
= 125°C
U
3.0 3.5
F
E
LTRA
Peak switch current
internal zener
0.9V
Voltage at pin 1
Diode - Forward voltage
0.7V at T
T
P
A
= -55°C
-L
R O D U C T I O N
4.0
OW
T H E O R Y O F O P E R A T I O N
V
Z
4.5
S
A
= 25 C
TART
1.1V
5.0
-U
P
C
URRENT
D
ERROR AMPLIFIER
The error amplifier has a PNP input differential stage with access
to the Inverting input and the output pin. The N.I. input is
internally biased to 2.5 volts and is not available for any external
connections. The maximum input bias current for the LX155XC
series is 0.5µA, while LX155XI/155XM devices are rated for 1µA
maximum over their specified range of ambient temperature.
Low value resistor dividers should be used in order to avoid
output voltage errors caused by the input bias current. The error
amplifier can source 0.5mA and sink 2mA of current. A minimum
feedback resistor (R
OUTPUT STAGE
The output section has been specifically designed for direct drive
of power MOSFETs. It has a totempole configuration which is
capable of high peak current for fast charging and discharging of
external MOSFET gate capacitance. This typically results in a rise
and fall time of 50ns for a 1000pf capacitive load. Each output
transistor (source and sink) is capable of supplying 200mA of
continuous current with typical saturation voltages versus tem-
perature as shown in Figures 21 & 22 of the characteristic curve
section. All devices are designed to minimize the amount of
shoot-thru current which is a result of momentary overlap of
output transistors. This allows more efficient usage of the IC at
higher frequencies, as well as improving the noise susceptibility
of the device. Internal circuitry insures that the outputs are held
off during V
section, shows output sink saturation voltage vs. current at 5V.
, C
A T A
URRENT
S
-M
CC
H E E T
ODE
R
ramp-up. Figure 20, in the characteristic curves
FMIN
PWM
F
=
) value of is given by:
3(1.1) + 1.8
0.5mA
LX1552/3/4/5
10K
15