NCP1550SN18T1 Datasheet, PDF (13/18 Pages) ON Semiconductor – 600 kHz PWM/PFM Step-Down DC-DC Controller

NCP1550
DETAILED OPERATING DESCRIPTION
Detailed Operating Description
The NCP1550 series are step−down (Buck) DC−DC
controllers designed primarily for use in portable
applications powered by battery cells. With an appropriate
external P−channel MOSFET connected, the device can
provide up to 2 A loading current with high conversion
efficiency. The NCP1550 series using an unique control
scheme combines the advantages of Pulse−Frequency−
Modulation (PFM) that can provide excellent efficiency
even at light loading conditions and Constant−Frequency
Pulse−Width−Modulation that can achieve high efficiency
and low output voltage ripple at heavy loads. The NCP1550
working at high switching frequency makes it possible to use
small size surface mount inductor and capacitors to reduce
PCB area and provide better interference handling for noise
sensitive applications. The simplified functional blocks of
the device are shown in Figure 1 and descriptions for each
of the functions are given below.
The Internal Oscillator
An oscillator that governs the switching of a PWM control
cycles is required. NCP1550 have an internal
Fixed−Frequency oscillator. The oscillator frequency is
trimmed to 600 kHz with an accuracy of ±15%. All other
timing signals needed for operation are derived from this
oscillator signal.
Voltage Reference and Soft−Start
An internal high accuracy voltage reference is included in
NCP1550. This reference voltage is connected to the
inverting input terminal of the error amplifier, A1, which
compared with portion of the output voltage, V_OUT derived
from an integrated voltage divider with precise trimming to
give the required output voltage at ±2% accuracy. NCP1550
also comes with a built−in soft−start circuit that controls the
ramping up of the internal reference voltage during the
power−up of the converter. This function effectively enables
the output voltage to rise gradually over the specified
soft−start time, 8 msec typical. This prevents the output
voltage from overshooting during start−up of the converter.
Voltage Mode Pulse−Width−Modulation (PWM)
Control Scheme
For normal operation, NCP1550 is working in
Constant−Frequency Pulse−Width−Modulation (PWM)
Voltage Mode Control. The controller operates with the
internal oscillator, which generates the required ramp
function to compare with the output of the error
amplifier, A1. The error amplifier compares the internally
divided−down output voltage with the voltage reference to
produce an error voltage at its output. This error voltage is
compared with the ramp function to generate the control
pulse to drive the external power switch. On a
cycle−by−cycle basis, the greater the error voltage, the
longer the switch is held on. Hence, corresponding
corrective action will be made to keep the output voltage
within regulation. Constant−Frequency PWM reduces
output voltage ripple and noise, which is one of the
important characteristics for noise sensitive communication
applications. The high switching frequency allows small
size surface mount components to improve layout
compactness and reduce PC board area, and eliminate audio
and emission interference.
Power−Saving Pulse−Frequency−Modulation (PFM)
Control Scheme
While the loading is decreasing, the converter enters the
Discontinuous Conduction Mode (DCM) operation, which
means the inductor current will decrease to zero before the
next switching cycle starts. In DCM operation, the ON time
for each switching cycle will decrease significantly when
the output current decreases. In order to maintain a high
conversion efficiency even at light load conditions, the ON
time for each switching cycle is closely monitored and for
any ON time smaller than the preset value, 320 nsec, the
switching pulse will be skipped. As a result, when the
loading current is small, the converter will be operating in a
“Constant ON time (320 nsec nominal), variable OFF time”
Pulse−Frequency Modulation (PFM) mode. This innovative
control scheme improves the conversion efficiency for the
system at light load and standby operating conditions hence
extend the operating life of the battery.
Low Power Shutdown Mode
NCP1550 can be disabled whenever the CE pin (Pin 1) is
tied to GND. In shutdown, the internal reference, oscillator,
control circuitry, driver and internal feedback voltage
divider are turned off and the output voltage falls to 0 V.
During the shutdown mode, as most of the internal functions
are stopped and current paths are cut−off, the device
consume extremely small current in this condition.
Under−Voltage Lockout (UVLO)
To prevent operation of the P−Channel MOSFET below
safe input voltage levels, an Undervoltage Lockout is
incorporated into the NCP1550. When the input supply
voltage drops below approximately 2.2 V, the comparator,
CP1 will turn−off the control circuitry and shut the converter
down.
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