ncp1510a ON Semiconductor, ncp1510a Datasheet - Page 10

• Manufacturer Part Number: ncp1510a
• Description: High Efficiency Dc-dc Converters
• 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
operation by connecting SYNC pin to V
the output 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 NCP1510A is a monolithic micro−power high
The DC−DC converter has two operating modes (normal
The NCP1510A can be set to current mode PWM
In continuous conduction mode (CCM), Q1 is turned ON
The overvoltage protection circuit is present in PWM
DETAILED OPERATING DESCRIPTION
CC
. In this mode,
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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)
can 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
is used to 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 NCP1510A
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