MC34280 Datasheet, PDF (11/20 Pages) ON Semiconductor – Power Supply & Management IC for Handheld Electronic Products

MC34280
DETAILED OPERATING DESCRIPTION (Cont’d)
2 x Iref
CMAINb
100 pF
RMAINb
1000 kOhm
VBAT
L1
33uH
1 VMAINFB
x2
0.5 V
Iref
IREF
8
RIref
480 kOhm
Voltage ^{1.22 V}
Reference
VCOMP
COMP1
AGND
31 VMAINSW
ZLC
COMP3
M2
+ve Edge Delay
for Max. ON Time
VDD
senseFET
RQ
M1
S Qb
DGND
1–SHOT
for Min. OFF Time
RQ
S
VDD
ILIM
DGND
COMP2
VMAIN
32 ₊
CMAIN
100 uF
VMAINGND
30
Voltage Reference
& Current Bias
Main Regulator
with Synchronous Rectifier
AGND
Figure 20. Simplified Block Diagram of Main Regulator
In Figure 22, regulator is operating at Discontinuous
Conduction Mode, waveforms are similar to those of Figure
21. However, coil current drops to zero before next
switching cycle starts.
To estimate conduction mode, below equation can be
used.
+ * Iroom
h T_ON Vin²
2 L Vout
ILOAD
where, η is efficiency, refer to Figure 6
if Iroom > 0, the regulator is at Discontinuous Conduction
mode
if Iroom = 0, the regulator is at Critical Conduction mode
where coil current just drops to zero and next cycle starts.
if Iroom < 0, the regulator is at Continuous Conduction
mode
ǒ Ǔ ⁺ * ^TSW
TON
1
h
Vin
Vout
(S);
ǒ Ǔ ⁺ * ^Ipk
ILOAD
1
TON
TSW
) Vin
2
TON
L
(A)
For Discontinuous Conduction mode, provided that
current limit is not reached,
ǒ Ǔ ⁺ @ @ ^@ @ _@ * ^TSW
Vin TO²_N
2 L ILOAD
Vout
h Vin
1
(S);
+ @ Ipk
Vin
L
TON
(A)
For Continuous Conduction mode, provided that current
limit is not reached,
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