FA13842 Datasheet, PDF (14/15 Pages) Fuji Electric – CMOS IC(For Switching Power Supply Control)

FA13842, 13843, 13844, 13845
Fig. 22 illustrates a case when the inductor current variation
∆ iL’ at t1 is smaller than ∆ iL at t0. In this case, inductor current
variations gradually converges and the inductor current
becomes stable.
It is necessary to apply slope compensation to the control
signals in order to prevent such subharmonic oscillations when
the inductor current is continuous and the duty cycle is greater
than 50%.
The waveform of the inductor current when slope
compensation is applied is shown in Fig. 23.
Slope compensation adds the negative slope of inclination
–Kc to the control signal of the inductor peak current.
∆ iL’ shows the variation of the inductor current at t1 when
slope compensation is not applied, and ∆ iL’ s shows the
variation of the inductor current at t1 when slope compensation
is applied.
Thus, ∆ iL’ can be changed by –Kc, and ∆ IL’ s becomes smaller
when –Kc is large. It is necessary to apply slope
compensation to satisfy the equation ∆ iL ≥ ∆ iL’s, that is,
I –Kc I ≥ I –1/2 Ld I as the condition which achieves stable
operation.
Typical circuits are shown in Fig. 24 and 25.
∆iL
to
t1
Fig. 22
Converge
∆iL´
-Kc
∆iL
Lu
Ton
T
to
Is
∆iL´
-Ld
∆iL´s
Compensated
t1
Fig. 23
Vcc
Vin
VCC
7
RT
Tr7
30V
UVLO
UVLO
Vcc _5VREF
ENB 2.5V
R27 CT
Output
R25
R24
RT/CT
4
FB
2
R26 C13
1
COMP
ISNS ³
ER AMP _2R
1R
OSC
1V
SQ
FF
R _QB
VREF
8
OUT
OUTPUT 6
ENB
R18
Rs
C10
5 GND
Fig. 24
Vcc
Vin
VCC
7
RT
Tr7
30V
UVLO
UVLO
Vcc _5VREF
ENB 2.5V
R27 ^CT
Output
R25
R24
RT/CT
4
FB
2
R26 C13
1
COMP
3
ISNS
ER AMP _2R
1R
OSC
1V
SQ
FF
R _QB
VREF
8
OUT
OUTPUT 6
ENB
R18
Rs
C10
5 GND
Fig. 25
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