NCP12700
www.onsemi.com
9
Application Information
The NCP12700 is a fixed frequency, peak current mode,
PWM controller containing all of the features necessary for
implementing single−ended power converter topologies.
The device features an ultra−wide range, high voltage
startup regulator capable of regulating V
CC
across an input
voltage range of 9 – 120 V (xDNR2G) or 9 – 200 V
(xMTTXG). The controller is designed for high speed
operation including a programmable oscillator capable of
operating from 100 kHz to 1 MHz and total propagation
delays less than 75 ns in the PWM path. The NCP12700
integrates slope compensation to prevent subharmonic
oscillations and an Input Voltage Compensation /
Over−Power Protection (OPP) feature that limits the
converter power delivery capability across input voltage,
easing system thermal design. The controller offers an
adjustable soft−start, input voltage UVLO protection, and a
suite of protection features including cycle−by−cycle
current limit and a FLT pin with a NTC interface for system
thermal protection. The UVLO pin also features a shutdown
comparator which allows for an externally applied
pull−down signal to disable switching and bring the
controller into a low quiescent state.
Ultra−Wide Range HV Startup Regulator
The NCP12700 features a high voltage startup regulator
capable of operating across input voltage ranges of 9−120 V
(xDNR2G) or 9−200 V (xMTTXG). The ultra−wide range
capability of the regulator allows for direct connection of
VIN to the converter input voltage without requiring
external components. The regulator’s input voltage
capabilities support a wide range of industrial, medical,
telecom, and transportation applications.
Figure 4 details the operation of the startup regulator.
When VIN is applied, the regulator will immediately begin
sourcing current to charge V
CC
. Initially the startup will
supply approximately 10 mA. Once V
CC
builds up to ~ 3 V,
the control loop for the HV regulator will activate and the
source current will be regulated to 30 mA until V
CC
reaches
the V
CC(REG)
level of 8 V. The HV startup is a linear
regulator which can continue to supply and regulate V
CC
at
8 V. The recommended V
CC
capacitance to ensure stability
of the regulator is 1 – 10 mF.
While the V
CC
voltage is below the V
CC(ON)
threshold the
controller will remain in a low quiescent state to allow for
rapid charging of V
CC
and fast startup of the application.
Once the V
CC
voltage reaches the V
CC(ON)
threshold,
approximately 200 mV below the V
CC(REG)
level, the
controller will exit the low quiescent state and begin
delivering drive pulses. While the output voltage is building
up, the startup regulator will continue to supply the current
necessary to maintain V
CC
at the V
CC(REG)
level. For low
input voltage applications, the startup regulator has been
designed to guarantee a minimum of 15 mA source
capability with 2 V of headroom.
In typical applications an auxiliary winding will be used
to provide bias to V
CC
once the converter is switching. This
allows for the most efficient operation of the system. Once
the auxiliary winding pulls the V
CC
voltage above
V
CC(REG)
, the HV regulator will shut off. In normal
operation the V
CC
voltage can be biased above the voltage
at VIN and can support voltages up to 28 V. A V
CC
OVP
protection feature will trigger at 28 V, disabling switching of
the converter to prevent the auxiliary winding voltage from
damaging the controller.
