NCP1593A, NCP1593B
http://onsemi.com
9
DETAILED DESCRIPTION
Overview
The NCP1593 is a synchronous PWM controller that
incorporates all the control and protection circuitry
necessary to satisfy a wide range of applications. The
NCP1593 employs current mode control to provide fast
transient response, simple compensation, and excellent
stability. The features of the NCP1593 include a precision
reference, fixed 1 MHz switching frequency, a
transconductance error amplifier, an integrated high−side
P−channel MOSFET and low−side N−Channel MOSFET,
internal soft−start, and very low shutdown current. The
protection features of the NCP1593 include internal
soft−start, pulse−by−pulse current limit, and thermal
shutdown.
Reference Voltage
The NCP1593 incorporates an internal reference that
allows output voltages as low as 0.6 V. The tolerance of the
internal reference is guaranteed over the entire operating
temperature range of the controller. The reference voltage is
trimmed using a test configuration that accounts for error
amplifier offset and bias currents.
Oscillator Frequency
A fixed precision oscillator is provided. The oscillator
frequency range is 1 MHz with $13% variation.
Transconductance Error Amplifier
The transconductance error amplifier’s primary function
is to regulate the converter’s output voltage using a resistor
divider connected from the converter’s output to the FB pin
of the controller, as shown in the applications schematic. If
a Fault occurs, the amplifier’s output is immediately pulled
to GND and PWM switching is inhibited.
Soft−Start
To limit the startup inrush current, a soft−start circuit is
used to ramp up the reference voltage from 0 V to its final
value linearly. This soft−start time is internally set to a typical
value of 500 ms, or it can be externally adjusted by adding a
capacitor (C
SS
) from the SS pin to GND. The following
formulas show how to set the externally adjustable soft-start
time. The maximum allowable C
SS
is 10 nF.
t
SS
+
ǒ
C
SS
V
FB
Ǔ
I
SS
(eq. 1)
Where:
V
FB
: Reference voltage, typically 0.6 V
I
SS
: Soft−start current, typically 0.7 mA
Output MOSFETs
The NCP1593 includes low R
DS(on)
, both high−side
P−channel and low−side N−channel MOSFETs capable of
delivering up to 3.0 A of current. When the controller is
disabled or during a Fault condition, the controller’s output
stage is tri−stated by turning OFF both the upper and lower
MOSFETs.
Pulse Width Modulation
A high−speed PWM comparator, capable of pulse widths
as low as 35 ns, is included in the NCP1593. The inverting
input of the comparator is connected to the output of the
error amplifier. The non−inverting input is connected to the
the current sense signal. At the beginning of each PWM
cycle, the CLK signal sets the PWM flip−flop and the upper
MOSFET is turned ON. When the current sense signal rises
above the error amplifier’s voltage then the comparator will
reset the PWM flip−flop and the upper MOSFET will be
turned OFF.
Current Sense
The NCP1593 monitors the current in the upper
MOSFET. The current signal is required by the PWM
comparator and the pulse−by−pulse current limiter.
