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NCP1521 Datasheet(PDF) 8 Page - ON Semiconductor

Part No. NCP1521
Description  1.5 MHz, 600 mA, High−Efficiency, Low Quiescent Current, Adjustable Output Voltage Step−Down Converter
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1521 Datasheet(HTML) 8 Page - ON Semiconductor

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NCP1521
http://onsemi.com
8
OPERATION DESCRIPTION
Overview
The NCP1521 uses a constant frequency, current mode
step−down architecture. Both the main (P−Channel
MOSFET) and synchronous (N−Channel MOSFET)
switches are internal.
It delivers a constant voltage from either a single Li−Ion
or three cell NiMH/NiCd battery to portable devices such
as cell phones and PDA. The output voltage is set by the
external resistor divider. The NCP1521 sources at least
600 mA, depending on external components chosen.
The NCP1521 works with two modes of operation;
PWM/PFM depending on the current required. The device
operates in PWM mode at load currents of approximately
40 mA or higher, having voltage tolerance of
"3% with
90% efficiency or better. Lighter load currents cause the
device to automatically switch into PFM mode for reduced
current consumption (IQ = 30
mA typ) and extended battery
life.
Additional features include soft−start, undervoltage
protection, current overload protection, and thermal
shutdown protection. As shown in Figure 1, only six
external components are required for implementation. The
part uses an internal reference voltage of 0.6 V. It is
recommended to keep the part in shutdown until the input
voltage is 2.7 V or higher.
PWM Operating Mode
In this mode, the output voltage of the NCP1521 is
regulated by modulating the on−time pulse width of the
main switch Q1 at a fixed frequency of 1.5 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.
Figure 25. PWM Switching Waveform
(Vin = 3.6 V, Vout = 1.8 V, Iout = 300 mA)
125 ns/div
PFM Operating Mode
Under light load conditions (<40 mA), the NCP1521
enters in low current PFM mode operation to reduce power
consumption. The output regulation is implemented by
pulse frequency modulation. If the output voltage drops
below the threshold of PFM comparator (typically
Vnom−2%), a new cycle will be initiated by the PFM
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 turned ON. The
negative current detector (NCD) will detect when the
inductor current drops below zero and sends 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 PFM comparator,
a new cycle starts immediately.
Figure 26. PFM Mode Switching Waveform
(Vin = 3.6 V, Vout = 1.8 V, Iout = 30 mA)
2.5
ms/div


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