NCP1521B
http://onsemi.com
9
OPERATION DESCRIPTION
Overview
The NCP1521B 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 an
external resistor divider. The NCP1521B sources at least
600 mA, depending on external components chosen.
The NCP1521B works with two modes of operation;
PWM/PFM depending on the current required. In PWM
mode, the device can supply voltage with a tolerance of
"3% and 90% efficiency or better. Lighter load currents
cause the device to automatically switch into PFM mode
for reduced current consumption 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. The part uses an internal
reference voltage of 0.6 V. It is recommended to keep the
part in shutdown mode until the input voltage is 2.7 V or
higher.
PWM Operating Mode
In this mode, the output voltage of the NCP1521B 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.
This driver switches ON and OFF the upper side transistor
(Q1) and switches the lower side transistor (Q2) in either
ON state or in current source mode. At the beginning of
each cycle, the main switch Q1 is turned ON while Q2 is
in its current source mode 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 in
its ON state. 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 27. PWM Switching Waveform
(V
IN
= 3.6 V, V
OUT
= 1.2 V, I
OUT
= 600 mA)
200 ns/div
V
OUT
10mV/div
I
Lx
100mA/div
V
Lx
2V/div
PFM Operating Mode
Under light load conditions, the NCP1521B 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, a new cycle will
be initiated by the PFM comparator to turn on the switch
Q1. Q1 remains ON during the minimum on time of the
structure while Q2 is in its current source mode. The peak
inductor current depends upon the drop between input and
output voltage. After a short dead time delay where Q1 is
switched OFF, Q2 is turned in its ON state. The negative
current detector will detect when the inductor current drops
below zero and sends the signal to turn Q2 to current source
mode to prevent a too large deregulation of the output
voltage. When the output voltage falls below the threshold
of the PFM comparator, a new cycle starts immediately.
Figure 28. PFM Mode Switching Waveform
(V
IN
= 3.6 V, V
OUT
= 1.2 V, I
OUT
= 0 mA)
V
out
10mV/div
I
Lx
100mA/div
V
Lx
2V/div
