CURRENT MODE PWM CONTROLLER
Fig.4 Current Sense Circuit
ERROR
AMP
I
S
2R
LM3842A/3A/4A/5A
1
R
COMP
CURRENT
SENSE
R
1V
CURRENT
SENSE
COMPARATOR
3
C
R
S
GND
5
Peak current (I
S
) is determined by the formula:
I
S(MAX)
~
1.0V
R
S
A small RC filter may be required to suppress switch transients.
Fig.5 Oscillator Waveforms and Maximum Duty Cycle
V
REF
8
R
T
R
T
/C
T
INTERNAL CLOCK
4
C
T
GND
5
INTERNAL CLOCK
LARGE R
T
SMALL C
T
V
4
LARGE R
T
SMALL C
T
V
4
Oscillator timing capacitor, C
T
, is charged by V
REF
through R
T
, and discharged by an internal
current source. During the discharge time, the internal clock signal blanks the output to the
low state. Selection of R
T
and C
T
therefore determines both oscillator frequency and maximum
duty cycle. Charge and discharge times are determined by the formulas:
t
c
~
0.55 R
T
C
T
~
t
d
~
R
T
C
T
∫n(
0.0063 R
T
- 2.7
)
0.0063 R
T
- 4
-1
Frequency, then, is: f = (t
c
+ t
d
)
For R
T
>5KΩ, f
~
1.8
R
T
C
T
HTC
4
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
