NCP1400A
http://onsemi.com
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DETAILED OPERATING DESCRIPTION
Operation
The NCP1400A series are monolithic power switching
regulators optimized for applications where power drain
must be minimized. These devices operate as fixed
frequency, voltage mode boost regulator and is designed to
operate in the discontinuous conduction mode. Potential
applications include low powered consumer products and
battery powered portable products.
The NCP1400A series are low noise fixed frequency
voltage–mode PWM DC–DC converters, and consist of
soft–start circuit, feedback resistor, reference voltage,
oscillator, loop compensation network, PWM control
circuit, current limit circuit and power switch. Due to the
on–chip feedback resistor and loop compensation network,
the system designer can get the regulated output voltage
from 1.8 V to 5.0 V with a small number of external
components. The quiescent current is typically 32 µA
(VOUT = 2.7 V), and can be further reduced to about 1.5 µA
when the chip is disabled (VCE 0.3 V).
Soft Start
There is a soft start circuit in NCP1400A. When power is
applied to the device, the soft start circuit pumps up the
output voltage to approximately 1.5 V at a fixed duty cycle,
the level at which the converter can operate normally. What
is more, the start–up capability with heavy loads is also
improved.
Oscillator
The oscillator frequency is internally set to 180 kHz at an
accuracy of 20% and with low temperature coef ficient o f
0.11%/°C. Figures 16 and 17 illustrate oscillator frequency
versus temperature.
Regulated Converter Voltage (VOUT)
The VOUT is set by an internal feedback resistor network.
This is trimmed to a selected voltage from 1.8 V to 5.0 V
range in 100 mV steps with an accuracy of 2.5%.
Compensation
The device is designed to operate in discontinuous
conduction mode. An internal compensation circuit was
designed to guarantee stability over the full input/output
voltage and full output load range. Stability cannot be
guaranteed in continuous conduction mode.
Current Limit
The NCP1400A series utilizes cycle–by–cycle current
limiting as a means of protecting the output switch
MOSFET from overstress and preventing the small value
inductor from saturation. Current limiting is implemented
by monitoring the output MOSFET current build–up during
conduction, and upon sensing an overcurrent conduction
immediately turning off the switch for the duration of the
oscillator cycle.
The voltage across the output MOSFET is monitored and
compared against a reference by the VLX limiter. When the
threshold is reached, a signal is sent to the PWM controller
block to terminate the output switch conduction. The current
limit threshold is typically set at 350 mA.
Enable/Disable Operation
The NCP1400A series offer IC shutdown mode by chip
enable pin (CE pin) to reduce current consumption. An
internal pull–up resistor tied the CE pin to OUT pin by
default, i.e., user can float the pin CE for permanent “On’’.
When voltage at pin CE is equal or greater than 0.9 V, the
chip will be enabled, which means the regulator is in normal
operation. W hen voltage at pin CE is less than 0.3 V, the chip
is disabled, which means IC is shutdown.
Important: DO NOT apply a voltage between 0.3 V to
0.9 V to pin CE as this voltage will place the IC into an
undefined state and the IC may drain excessive current
from the supply.