NCP1403_05 ONSEMI [ON Semiconductor], NCP1403_05 Datasheet - Page 11

• Manufacturer Part Number: NCP1403_05
• Description: 15 V/50 mA PFM Step−Up DC−DC Converter
• Manufacturer: ONSEMI [ON Semiconductor]

Output Voltage Higher than 15 V
than 15 V by adding an external high voltage N−Channel
MOSFET in series with the internal MOSFET switch as
shown in Figure 33. The drain−to−source breakdown
voltage of the external MOSFET must be at least 1 V higher
than the output voltage. The diode D1 helps the external
MOSFET to turn off and ensures that most of the voltage
across the external MOSFET during the switch−off period.
Since the high voltage external MOSFET is in series with the
internal MOSFET, higher break down voltage is achieved
but the current capability is not increased.
35 which can output voltage up to 30 V. For this circuit, a
diode−capacitor charge−pump voltage doubler constructed
by D2, D3 and C1 is added. During the internal MOSFET
switch−on time, the LX pin is shorted to ground and D2 will
charge up C1 to the stepped up voltage at the cathode of D1.
During the MOSFET switch−off time, the voltage at V
will be almost equal to the double of the voltage at the
cathode of D1. The V
resistor divider and can be set by the resistor values. Since
the maximum voltage at the cathode of D1 is 15 V, the
maximum V
of 0.47 mF to 2.2 mF.
Negative Voltage Generation
output by adding a diode−capacitor charge−pump circuit
(D2, D3, and C1) to the LX pin as shown in Figure 32. The
feedback voltage resistor divider is still connected to the
positive output to monitor the positive output voltage and a
small value capacitor is used at C2. When the internal
MOSFET switches off, the voltage at the LX pin charges up
the capacitor through diode D2. When the MOSFET
switches on, the capacitor C1 is effectively connected like a
reversed battery and C1 discharges the stored charges
through the R
charge up C
Since the negative voltage output is not directly monitored
by the NCP1403, the output load regulation of the negative
output is not as good as the standard positive output circuit.
The resistance values of the resistors of the voltage divider
can be one−tenth of those used in the positive output circuit
in order to improve the regulation at light load.
combination of the application circuits in Figures 32 and 33.
Step−Down Converter
converter by using the open−drain LX pin to drive an
external P−Channel MOSFET as shown in Figure 34. The
resistor RGS is used to switch off the P−Channel MOSFET
during the switch−off period. Too small resistance value
should not be used for R
reduced.
NCP1403 can be used to generate output voltage higher
There is an alternative application circuit shown in Figure
The NCP1403 can be used to produce a negative voltage
For the application circuit in Figure 36, it is actually the
NCP1403 can be configured as a simple step−down
OUT
OUT
ds(on)
and builds up a negative voltage at V
is 30 V. The value of C1 can be in the range
of the internal MOSFET and D3 to
OUT
GS
is monitored by the FB pin via the
, otherwise, the efficiency will be
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OUT
OUT
NCP1403
.
11
White LED Driver
driver which can drive up to 4 white LEDs in series as shown
in Figure 2. The LED current can be set by the resistance
value of RS. The desired LED current can be calculated by
the equation below:
a DC voltage or a PWM signal with an additional circuit
illustrated below:
set by the above equation. The value of R2 can be obtained
by the following equation:
is the diode forward voltage, I
LED current and I
a PWM control signal is used, the signal frequency from 4
kHz to 40 kHz can be applied.
zero. The NCP1403 will then switch at maximum duty cycle
and result in a high output voltage which will cause the LX
pin voltage to exceed its maximum rating. A Zener diode can
be added across the output and FB pin to limit the voltage at
the LX pin. The Zener voltage should be higher than the total
forward voltage of the LED string.
PCB Layout Hints
placement of the step−up DC−DC converter demonstration
board are shown in Figure 28 to Figure 31 for PCB layout
design reference.
Grounding
return ground, the input power return ground, and the device
switch ground to reduce noise. The input ground and output
ground traces must be thick and short enough for current to
flow through. A ground plane should be used to reduce
ground bounce.
The NCP1403 can be used as a constant current LED
Moreover, the brightness of the LEDs can be adjusted by
With this additional circuit, the maximum LED current is
V
In case the LEDs fail, the feedback voltage will become
The schematic, PCB trace layout, and component
One point grounding should be used for the output power
DC/PWM
Signal
MAX
is the maximum voltage of the control signal, V
R2 +
GND
LED(MIN)
0.1 mF
C1
R2
(I LED(MAX) *I LED(MIN) ) R S
V MAX * V D * 0.8
I LED + 0.8
is the minimum LED current. If
D2
To FB Pin
R1
LED(MAX)
R S
820 pF
100 k
R1
C2
is the maximum
To LED
R
S
D