VLCF4020-6R8 ONSEMI [ON Semiconductor], VLCF4020-6R8 Datasheet - Page 10

• Manufacturer Part Number: VLCF4020-6R8
• Description: Up to 500 mA, High Efficiency Synchronous Step-Down DC-DC Converter
• Manufacturer: ONSEMI [ON Semiconductor]

Overview
frequency PWM step−down DC−DC converter specifically
optimized for applications requiring high efficiency and a
small PCB footprint such as portable battery powered
products. It integrates synchronous rectification to
improve efficiency as well as eliminate the external
Schottky diode. High switching frequency allows for a low
profile inductor and capacitors to be used. Four digital
selectable output voltages (1.0, 1.3, 1.5 and 1.89 V) can be
generated from the input supply that can range from
2.7−5.2 V. All loop compensation is integrated as well
further reducing the external component count as well.
PWM, pulsed switching), which are intended to allow for
optimum efficiency under either light (up to 30 mA) or
heavy loads. The user determines the operating mode by
controlling the SYNC input. In addition the SYNC input
can be used to synchronize the PWM to an external system
clock signal in the range of 500−1000 kHz.
PWM Operating Mode
by connecting SYNC pin to V
voltage is regulated by modulating the on−time pulse width
of the main switch Q1 at a fixed frequency of 1.0 MHz. The
switching of the PMOS Q1 is controlled by a flip−flop
driven by the internal oscillator and a comparator that
compares the error signal from an error amplifier with the
sum of the sensed current signal and compensation ramp.
At the beginning of each cycle, the main switch Q1 is
turned ON by the rising edge of the internal oscillator
clock. The inductor current ramps up until the sum of the
current sense signal and compensation ramp becomes
higher than the error voltage amplifier. Once this has
occurred, the PWM comparator resets the flip−flop, Q1 is
turned OFF and the synchronous switch Q2 is turned ON.
Q2 replaces the external Schottky diode to reduce the
conduction loss and improve the efficiency. To avoid
overall power loss, a certain amount of dead time is
introduced to ensure Q1 is completely turned OFF before
Q2 is being turned ON.
after Q2 is completely turned OFF to start a new clock
cycle. In discontinuous conduction mode (DCM), the zero
crossing comparator (ZLC) will turn off Q2 when the
inductor current drops to zero.
Overvoltage Protection
mode to prevent the output voltage from going too high
under light load or fast load transient conditions. The
output overvoltage threshold is 5% above nominal set
value. If the output voltage rises above 5% of the nominal
The NCP1511 is a monolithic micro−power high
The DC−DC converter has two operating modes (normal
The NCP1511 can be set to current mode PWM operation
In continuous conduction mode (CCM), Q1 is turned ON
The overvoltage protection circuit is present in PWM
CC
. In this mode, the output
DETAILED OPERATING DESCRIPTION
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NCP1511
10
value, the OVP comparator is activated and switch Q1 is
turned OFF. Switching will continue when the output
voltage falls below the threshold of OVP comparator.
Pulsed Mode (PM)
be configured to enter a low current pulsed mode operation
to reduce power consumption. This is accomplished by
applying a logic LOW to the SYNC pin. The output
regulation is implemented by pulse frequency modulation.
If the output voltage drops below the threshold of PM
comparator (typically Vnom−2%), a new cycle will be
initiated by the PM comparator to turn on the switch Q1. Q1
remains ON until the peak inductor current reaches 200 mA
(nom). Then ILIM comparator goes high to switch off Q1.
After a short dead time delay, switch rectifier Q2 is turn
ON. The zero crossing comparator will detect when the
inductor current drops to zero and send the signal to turn off
Q2. The output voltage continues to decrease through
discharging the output capacitor. When the output voltage
falls below the threshold of the PM comparator again, a
new cycle starts immediately.
Cycle−by−Cycle Current Limit
realize cycle−by−cycle current limit protection. The
comparator compares the LX pin voltage with the
reference voltage from the SENFET, which is biased by a
constant current. If the inductor current reaches the limit,
the ILIM comparator detects the LX voltage falling below
the reference voltage from the SENFET and releases the
signal to turn off the switch Q1. The cycle−by−cycle
current limit is set at 800 mA (nom) in PWM and 200 mA
in PM.
Frequency Synchronization and Operating Mode
Selection
synchronization by connecting it with an external clock
signal. It operates in PWM mode when synchronized to an
external clock. The switching cycle initiates by the rising
edge of the clock. The 500 kHz to 1000 kHz
synchronization clock signal should be between 0.4 V and
1.2 V.
used to select between PM and PWM modes. It allows
efficient dynamical power management by adjusting the
converter operation to the specific system requirement. Set
SYNC pin low to select PM mode at light load conditions
(up to 30 mA) and set SYNC pin high or connect with
external clock to select PWM mode at heavy load condition
to achieve optimum efficiency. Table 1 shows the mode
selection with three different SYNC pin states.
Under light load conditions (< 30 mA), the NCP1511 can
From the block diagram, an ILIM comparator is used to
The SYNC pin can also be used for frequency
Gating on and off the clock, the SYNC pin can also be