NCP1205 ON Semiconductor, NCP1205 Datasheet - Page 9

• Manufacturer Part Number: NCP1205
• Description: Single Ended Pwm Controller Featuring Qr Operation And Soft Frequency Foldback
• Manufacturer: ON Semiconductor

Introduction
technique, the NCP1205 represents a major leap toward
low−power Switchmode Power Supply (SMPS) integrated
management. The circuit combines free−running operation
with minimum drain−source switching (so−called valley
switching), which naturally reduces the peak current stress
as well as the ElectroMagnetic Interferences (EMI). At
switching frequency raises. As a natural result, switching
losses also increase and degrade the SMPS efficiency. To
overcome this problem, the maximum switching frequency
of the NCP1205 is clamped to typically 125 kHz. When the
free running mode (also called Borderline Control Mode,
BCM)
Voltage−Controlled Oscillator (VCO) takes over and starts
to decrease the switching frequency: we are in Variable
Frequency Mode (VFM). Please note that during this
transition phase, the peak current is not fixed but is still
decreasing because the output power demand does. At a
given state, the peak current reaches a minimum peak
(typically 250 mV/Rsense), and cannot go further down: the
switching frequency continues its decrease down to a
possible minimum of 0 Hz (the IC simply stops switching).
During normal free−running operation and VFM, the
controller always ensures single or multiple drain−source
By implementing a unique smooth frequency reduction
When the output power demands decreases, the natural
reaches
Figure 10. Defining the Conduction Mode, Discontinuous, Continuous and Borderline
this
clamp
I
L(avg)
I
L
value,
ON
Borderline
APPLICATION INFORMATION
an
D/Fs
I
P
internal
http://onsemi.com
Turn ON
Not 0 at
OFF
Time
L = Lc
Dead−Time
9
nominal output power, the circuit implements a traditional
current−mode SMPS whose peak current setpoint is given
by the feedback signal. However, rather than keeping the
switching frequency constant, each cycle is initiated by the
end of the primary demagnetization. The system therefore
operates at
Conduction Mode (DCM) and Continuous Conduction
Mode (CCM). Figure 10 details this terminology:
valley switching. We will see later on how this is internally
implemented.
simple formula:
With:
Lp the primary transformer inductance (also called the
magnetizing inductance)
Ip the peak current at which the MOSFET is turned off
Fsw the nominal switching frequency
play on the switching frequency or the peak current setpoint.
To refine the control, the NCP1205 offers the ability to play
on both parameters either altogether on an individual basis.
The FLYBACK operation is mainly defined through a
To adjust the transmitted power, the PWM controller can
0 Before
Turn ON
L > Lc
the
Pout + 1
L < Lc
boundary
2
· Lp · Ip 2 · Fsw
0
0
between
Discontinuous
(eq. 1)