ncp1509 ON Semiconductor, ncp1509 Datasheet - Page 10

• Manufacturer Part Number: ncp1509
• Description: Up To 500 Ma, 1 Mhz, High Efficiency Synchronous Step-down Dc-dc Converter In Dfn Package
• Manufacturer: 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 (1.05, 1.35, 1.57 and 1.8 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 on−time pulse width of
the main switch Q1 at a fixed frequency of 1 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 oscillation 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 value, the OVP
comparator is activated and switch Q1 is turned OFF.
The NCP1509 is a monolithic micro−power high
The DC−DC converter has two operating modes (normal
The NCP1509 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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Switching will continue when the output voltage falls below
the threshold of OVP comparator.
Pulsed Mode (PM)
configured to enter a low current pulsed mode operation to
reduce power consumption. This is accomplished by SYNC
pin held LOW. 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
200 mA inductor peak current is reached. 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 PM comparator again, a new cycle starts immediately.
Cycle−by−Cycle Current Limit
is used to realize cycle−by−cycle current limit protection.
The comparator compares the LX pin voltages with the
reference voltage from the SENFET, which is biased by
constant current. If the inductor current reaches the limit,
ILIM comparator detects the LX voltage falling below the
reference voltage from SENFET and releases the signal to
turn off the switch Q1. The cycle−by−cycle current limit is
set at 800 mA 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 synchronization clock signals
between 0.4 V and 1.2 V from 500 kHz to 1000 kHz.
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), NCP1509 can be
From the block diagram (Figure 3), an ILIM comparator
The SYNC pin can also be used for frequency
Gating on and off the clock, the SYNC pin can also be used