NCP1546
BOOST Pin
The BOOST pin provides base driving current for the
power switch. A voltage higher than V
IN
provides required
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
VBOOST
+
VIN
)
VO
*
VF
The IC enters a sleep mode when the SHDNB pin is pulled
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
CC
with a resistor (See Figure 1).
Startup
where:
V
F
= diode forward voltage.
The anode of the diode can be connected to any DC
voltage as well as the regulated output voltage (Figure 1).
However, the maximum voltage on the BOOST pin shall not
exceed 40 V.
As shown in Figure 7, the BOOST pin current includes a
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.
30
BOOST PIN CURRENT (mA)
25
20
15
10
5
0
During power up, the regulator tends to quickly charge up
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
2
control , the
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
C
pin and thus output
voltage ramp up gradually. The Soft−Start duration can be
calculated by
V
CCOMP
TSS
+
C
ISOURCE
where:
V
C
= V
C
pin steady−state voltage, which is approximately
equal to error amplifier’s reference voltage.
C
COMP
= Compensation capacitor connected to the V
C
pin
I
SOURCE
= Output Source Current of the error amplifier.
Using a 0.1
mF
C
COMP
, the calculation shows a T
SS
over
5.0 ms which is adequate to avoid any current stresses.
Figure 8 shows the gradual rise of the V
C
, V
O
and envelope
of the V
SW
during power up. There is no voltage over−shoot
after the output voltage reaches the regulation. If the supply
voltage rises slower than the V
C
pin, output voltage may
over−shoot.
0
0.5
1.0
SWITCHING CURRENT (A)
1.5
Figure 7. The Boost Pin Current Includes 7.0 mA
Pre−Driver Current and Base Current when the
Switch is Turned On. The Beta Decline of the
Power Switch Further Increases the Base
Current at High Switching Current
Shutdown
The internal power switch will not turn on until the V
IN
pin rises above the Startup Voltage. This ensures no
switching will occur until adequate supply voltage is
provided to the IC.
Figure 8. The Power Up Transition of NCP1546
Regulator
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