ncv8925 ON Semiconductor, ncv8925 Datasheet - Page 10

• Manufacturer Part Number: ncv8925
• Description: 3 Mhz, 600 Ma Step-down Dc-dc Converter
• Manufacturer: ON Semiconductor

Soft-Start
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
realize cycle-by-cycle current limit protection. The
comparator compares the LX pin voltage with the
reference voltage, which is biased by a constant current. If
the inductor current reaches the limit, the I
detects the LX 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 1200 mA (nom).
Low Dropout Operation
difference. The NCV8925 can operate at 100% duty cycle.
In this mode the PMOS (Q1) remains completely on.
be calculated as:
•
•
•
•
The NCV8925 uses soft-start to limit the inrush current
From the block diagram, an I
The NCV8925 offers a low input to output voltage
The minimum input voltage to maintain regulation can
V
I
R
R
OUT
V IN(min) + V OUT(max)
OUT
DS(ON)
INDUCTOR
: Max Output Current
: Output Voltage (Volts)
= P-Channel Switch R
: Inductor Resistance (DCR)
) (I OUT
(R DS(ON) ) R INDUCTOR ))
LIM
DS(ON)
comparator is used to
LIM
comparator
http://onsemi.com
(eq. 1)
NCV8925
10
Undervoltage Lockout
NCV8925 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
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 device for normal operation. The typical
threshold is around 0.7 V. The device will go through
soft-start to normal operation.
Thermal Shutdown
protect the integrated circuit in the event that the maximum
junction temperature is exceeded. If the junction
temperature exceeds 160°C, the device shuts down. In this
mode switch Q1 and Q2 and the control circuits are all
turned off. The device restarts in soft-start after the
temperature drops below 135°C. This feature is provided
to prevent catastrophic failures from accidental device
overheating, and it is not intended as a substitute for proper
heatsinking.
Short Circuit Protection
the inductor current. The duty-cycle is minimum and the
consumption on the input line is 300 mA (Typ). When the
short circuit condition is removed, the device returns to the
normal mode of operation.
The input voltage V
Forcing this pin to a voltage below 0.4 V will shut down
Internal Thermal Shutdown circuitry is provided to
When the output is shorted to ground, the device limits
IN
must reach 2.4 V (typ) before the