MC44603 Datasheet, PDF (14/24 Pages) ON Semiconductor – MIXED FREQUENCY MODE GREENLINE PWM CONTROLLER

MC44603
established by the Error Amplifier output (Pin 13). Thus, the
error signal controls the peak inductor current on a
cycle–by–cycle basis. The Current Sense Comparator PWM
Latch ensures that only a single pulse appears at the Source
Output during the appropriate oscillator cycle.
The inductor current is converted to a voltage by inserting
the ground referenced sense resistor RS in series with the
power switch Q1.
This voltage is monitored by the Current Sense Input
(Pin 7) and compared to a level derived from the Error Amp
output. The peak inductor current under normal operating
conditions is controlled by the voltage at Pin 13 where:
[ Ipk
V(Pin _{13) – 1.4 V}
3 RS
The Current Sense Comparator threshold is internally
clamped to 1.0 V. Therefore, the maximum peak switch
current is:
[ Ipk(max)
1.0 V
RS
Figure 34. Oscillator
Vref
0.4 Iref
1.0 V
10
CT
1.6 V
3.6 V
CVOS _prot
VOSC _prot
COSC _Low
CT < 1.6 V
Discharge
COSC _High
RQ
Disch
S
RQ
LOSC
S
COSC _Regul
VOSC
Synchro
VDemag
Out
01
10
IRegul
IDischarge
Figure 35. Simplified Block Oscillator
Figure 33. Output Totem Pole
Vin
VC
UVLO
VOSC _prot
VDemag _Out
Thermal
Protection
S
RQ
R
14
R2
Q1
3
R3
1N5819
PWM
Latch
Current Sense
Comparator
Current
Substrate _Sense
R
7
C
RS
Oscillator
The oscillator is a very accurate sawtooth generator that
can work either in free mode or in synchronization mode. In
this second mode, the oscillator stops in the low state and
waits for a demagnetization or a synchronization pulse to
start a new charging cycle.
• The Sawtooth Generation:
In the steady state, the oscillator voltage varies between
about 1.6 V and 3.6 V.
The sawtooth is obtained by charging and discharging an
external capacitor CT (Pin 10), using two distinct current
sources = Icharge and Idischarge. In fact, CT is permanently
connected to the charging current source (0.4 Iref) and so,
the discharge current source has to be higher than the
charge current to be able to decrease the CT voltage (refer
to Figure 35).
This condition is performed, its value being (2.0 Iref) in
normal working and (0.4 Iref + 0.5 IF _Stby in standby mode).
Vref
ICharge
0.4 Iref
10
COSC _Regul
1.6 V
0 1 _{0: Discharge Phase}
1: Charge Phase
CT
IDischarge
IRegul
Two comparators are used to generate the sawtooth. They
compare the CT voltage to the oscillator valley (1.6 V) and
peak reference (3.6 V) values. A latch (Ldisch) memorizes the
oscillator state.
In addition to the charge and discharge cycles, a third
state can exist. This phase can be produced when, at the end
of the discharge phase, the oscillator has to wait for a
synchronization or demagnetization pulse before restarting.
] ^{During this delay, the CT voltage must remain equal to the}
oscillator valley value ( 1.6 V). So, a third regulated current
source IRegul controlled by COSC _Regul, is connected to CT in
order to perfectly compensate the (0.4 Iref) current source
that permanently supplies CT.
The maximum duty cycle is 80%. Indeed, the on–time is
allowed only during the oscillator capacitor charge.
Consequently:
Tcharge = CT x ∆V/Icharge
Tdischarge = CT x ∆V/Idischarge
where:
Tcharge is the oscillator charge time
∆V is the oscillator peak–to–peak value
Icharge is the oscillator charge current
and
Tdischarge is the oscillator discharge time
Idischarge is the oscillator discharge current
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MOTOROLA ANALOG IC DEVICE DATA