NCP4328
www.onsemi.com
7
TYPICAL CHARACTERISTICS
T
J
(°C)
1.40
f
SWLED
(kHz)
−40 −20 0 20 40 60 80 100 120
Figure 13. LED Switching Frequency at
V
CC
= 15 V
T
J
(°C)
100
R
SW2
(W)
−40 −20 0 20 40 60 80 100 120
1.30
1.20
1.10
1.00
0.90
0.80
90
80
70
60
50
40
30
Figure 14. R
SW
at V
CC
= 15 V
APPLICATION INFORMATION
Typical application circuit for NCP4328A is shown in
Figures 15 and 16 shows typical application circuit for
NCP4328B that includes internal LED driver for indication
purpose.
Power Supply
The NCP4328 is designed to operate from a single supply
up to 36 V. It starts to operate when VCC voltage reaches
3.5 V and stops when VCC voltage drops below 2.5 V. VCC
can be supplied by direct connection to the VOUT voltage
of the power supply. It is highly recommended to add a RC
filter (R1 and C2) in series from VOUT to VCC pin to reduce
voltage spikes and drops that are produced at the converter’s
output capacitors. Recommended values for this filter are
220 W and 1 mF.
Voltage Regulation Path
The output voltage is detected on the VSNS pin by the R3
and R4 voltage divider. This voltage is compared with the
internal precise voltage reference. The voltage difference is
amplified by gm
V
of the transconductance amplifier. The
amplifier output current is connected to the FBC pin. The
compensation network is also connected to this pin to
provide frequency compensation for the voltage regulation
path. This FBC pin drives regulation optocoupler that
provides regulation of primary side. The optocoupler is
supplied via direct connection to VOUT line through
resistor R2.
Regulation information is transferred through the
optocoupler to the primary side controller where its FB pin
is usually pulled down to reduce energy transferred to
secondary output.
The output voltage can be computed by Equation 1.
V
OUT
+ V
REF
R3 ) R4
R4
(eq. 1)
Current Regulation
The output current is sensed by the shunt resistor R5 in
series with the load. Voltage drop on R5 is compared with
internal precise voltage reference V
REFC
at I
SNS
transconductance amplifier input.
Voltage difference is amplified by gm
C
to output current
of amplifier, connected to FBC pin. Compensation network
is connected between this pin and ISNS input to provide
frequency compensation for current regulation path.
Resistor R6 separates compensation network from sense
resistor. Compensation network works into low impedance
without this resistor that significantly decreases
compensation network impact.
Current regulation point is set to current given by
Equation 2.
I
OUTLIM
+
V
REFC
R5
(eq. 2)
LED Driver (NCP4328B only)
LED driver is active when VCC is higher than V
CCMIN
.
LED driver consists of an internal power switch controlled
by a PWM modulated logic signal and an external current
limiting resistor R9. LED current can be computed by
Equation 3
I
LED
+
V
OUT
* V
F_LED
R9
(eq. 3)
PWM modulation is used to increase efficiency of LED.
