NCP1529
http://onsemi.com
11
Soft−Start
The NCP1529 uses soft−start to limit the inrush current
when the device is initially powered up or enabled. Soft start
is implemented by gradually increasing the reference
voltage until it reaches the full reference voltage. During
startup, a pulsed current source charges the internal
soft−start capacitor to provide gradually increasing
reference voltage. When the voltage across the capacitor
ramps up to the nominal reference voltage, the pulsed
current source will be switched off and the reference voltage
will switch to the regular reference voltage.
Cycle−by−cycle Current Limitation
From the block diagram, an I
LIM
comparator is used to
realize cycle−by−cycle current limit protection. The
comparator compares the SW pin voltage with the reference
voltage, which is biased by a constant current. If the inductor
current reaches the limit, the I
LIM
comparator detects the
SW voltage falling below the reference voltage and releases
the signal to turn off the switch Q1. The cycle−by−cycle
current limit is set at 1600 mA (nom).
Low Dropout Operation
The NCP1529 offers a low input to output voltage
difference. The NCP1529 can operate at 100% duty cycle.
In this mode the PMOS (Q1) remains completely ON. The
minimum input voltage to maintain regulation can be
calculated as:
V
out
+ V
OUT(max)
)
ǒ
I
OUT
ǒ
R
DS(on)
_R
INDUCTOR
Ǔ
Ǔ
(eq. 1)
• V
OUT
: Output Voltage (V)
• I
OUT
: Max Output Current
• R
DS(on)
: P−Channel Switch R
DS(on)
• R
INDUCTOR
: Inductor Resistance (DCR)
Undervoltage Lockout
The Input voltage V
IN
must reach 2.4 V (typ) before the
NCP1529 enables the DC/DC converter output to begin the
start up sequence (see soft−start section). The UVLO
threshold hysteresis is typically 100 mV.
Shutdown Mode
Forcing this pin to a voltage below 0.4 V will shut down
the IC. In shutdown mode, the internal reference, oscillator
and most of the control circuitries are turned off. Therefore,
the typical current consumption will be 0.3 mA (typical
value). Applying a voltage above 1.2 V to EN pin will enable
the DC/DC converter for normal operation. The device will
go through soft−start to normal operation.
Thermal Shutdown
Internal Thermal Shutdown circuitry is provided to
protect the integrated circuit in the event that the maximum
junction Temperature is exceeded. If the junction
temperature exceeds 180°C, the device shuts down. In this
mode all power transistors and control circuits are turned
off. The device restarts in soft−start after the temperature
drops below 140°C. This feature is provided to prevent
catastrophic failures from accidental device overheating.
Short Circuit Protection
When the output is shorted to ground, the device limits the
inductor current. The duty−cycle is minimum and the
consumption on the input line is 550 mA (typ). When the
short circuit condition is removed, the device returns to the
normal mode of operation.
USB or 5 V Rail Powered Applications
For USB or 5 V rail powered applications, NCP1529 is
able to supply voltages up to 3.9 V, 600 mA, operating in
PWM mode only, with high efficiency (Figure 28), low
output voltage ripple and good load regulation results over
all current range (Figure 29).
40
45
50
55
60
65
70
75
80
85
90
95
100
0 200 400 600 800 1000
EFFICIENCY (%)
I
OUT
, OUTPUT CURRENT (mA)
Figure 28. Efficiency vs. Output Current
(V
IN
= 5.0 V, V
OUT
= 3.9 V)
25°C
85°C
−40°C
−3.0
−2.5
−2.0
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 200 400 600 800 1000
LOAD REGULATION (%)
I
OUT
, OUTPUT CURRENT (mA)
Figure 29. Load Regulation vs. Output Current
(V
IN
= 5.0 V, V
OUT
= 3.9 V)
25°C
85°C
−40°C
