NCP1546 ON Semiconductor, NCP1546 Datasheet - Page 8

• Manufacturer Part Number: NCP1546
• Description: Buck Regulator
• Manufacturer: ON Semiconductor

BOOST Pin
power switch. A voltage higher than V
headroom to turn on the power switch. This in turn reduces
IC power dissipation and improves overall system
efficiency. The BOOST pin can be connected to an external
boost−strapping circuit which typically uses a 0.1 mF capacitor
and a 1N914 or 1N4148 diode, as shown in Figure 1. When the
power switch is turned on, the voltage on the BOOST pin is
equal to
where:
voltage as well as the regulated output voltage (Figure 1).
However, the maximum voltage on the BOOST pin shall not
exceed 40 V.
constant 7.0 mA pre−driver current and base current
proportional to switch conducting current. A detailed
discussion of this current is conducted in Thermal
Consideration section. A 0.1 mF capacitor is usually
adequate for maintaining the Boost pin voltage during the on
time.
Shutdown
pin rises above the Startup Voltage. This ensures no
switching will occur until adequate supply voltage is
provided to the IC.
The BOOST pin provides base driving current for the
V
The anode of the diode can be connected to any DC
As shown in Figure 7, the BOOST pin current includes a
The internal power switch will not turn on until the V
30
25
15
10
20
F
5
0
Figure 7. The Boost Pin Current Includes 7.0 mA
Pre−Driver Current and Base Current when the
= diode forward voltage.
0
Switch is Turned On. The Beta Decline of the
Power Switch Further Increases the Base
Current at High Switching Current
V BOOST + V IN ) V O * V F
SWITCHING CURRENT (A)
0.5
IN
1.0
provides required
http://onsemi.com
1.5
NCP1546
IN
8
below the Shutdown Threshold Voltage. In sleep mode, the
power switch is kept open and the supply current reduces to
Shutdown Quiescent Current ( 1 mA typically). This pin has
an internal pull−down current. When not in use, pull this pin
up to V
Startup
the output capacitors to reach voltage regulation. This gives
rise to an excessive in−rush current which can be detrimental
to the inductor, IC and catch diode. In V
compensation capacitor provides Soft−Start with no need
for extra pin or circuitry. During the power up, the Output
Source Current of the error amplifier charges the
compensation capacitor which forces V
voltage ramp up gradually. The Soft−Start duration can be
calculated by
where:
5.0 ms which is adequate to avoid any current stresses.
Figure 8 shows the gradual rise of the V
of the V
after the output voltage reaches the regulation. If the supply
voltage rises slower than the V
over−shoot.
The IC enters a sleep mode when the SHDNB pin is pulled
During power up, the regulator tends to quickly charge up
V
C
I
Using a 0.1 mF C
SOURCE
COMP
C
Figure 8. The Power Up Transition of NCP1546
= V
CC
equal to error amplifier’s reference voltage.
SW
C
= Compensation capacitor connected to the V
with a resistor (See Figure 1).
pin steady−state voltage, which is approximately
during power up. There is no voltage over−shoot
= Output Source Current of the error amplifier.
T SS +
COMP
Regulator
, the calculation shows a T
V C
I SOURCE
C COMP
C
pin, output voltage may
C
C
pin and thus output
, V
O
2
and envelope
control , the
SS
C
over
pin