MC34280 Datasheet, PDF (13/20 Pages) ON Semiconductor – Power Supply & Management IC for Handheld Electronic Products

MC34280
DETAILED OPERATING DESCRIPTION (Cont’d)
Synchronous Rectification
A Synchronous Rectifier is used in the main regulator to
enhance efficiency. Synchronous rectifier is normally
realized by powerFET with gate control circuitry which,
however, involved relative complicated timing concerns. In
Figure 20, as main switch M1 is being turned OFF, if the
synchronous switch M2 is just turned ON with M1 not being
completed turned OFF, current will be shunt from the output
bulk capacitor through M2 and M1 to ground. This power
loss lowers overall efficiency. So a certain amount of dead
time is introduced to make sure M1 is completely OFF
before M2 is being turned ON, this timing is indicated as tdh
in Figure 21.
When the main regulator is operating in continuous mode,
as M2 is being turned OFF, and M1 is just turned ON with
M2 not being completed OFF, the above mentioned situation
will occur. So dead time is introduced to make sure M2 is
completed OFF before M1 is being turned ON, this is
indicated as tdl in Figure 21.
When the main regulator is operating in discontinuous
mode, as coil current is dropped to zero, M2 is supposed to
be OFF. Fail to do so, reverse current will flow from the
output bulk capacitor through M2 and then the inductor to
the battery input. It causes damage to the battery. So
M2–voltage–drop sensing comparator (COMP3 of Figure
20) comes with fixed offset voltage to switch M2 OFF
before any reverse current builds up. However, if M2 is
switch OFF too early, large residue coil current flows
through the body diode of M2 and increases conduction loss.
Therefore, determination on the offset voltage is essential
for optimum performance.
Auxiliary Regulator
The Auxiliary Regulator is a boost regulator, applies PFM
scheme to enhance high efficiency and reduce quiescent
current. An internal voltage comparator (COMP1 of Figure
23) detects when the voltage of Pin VAUXFBN drops below
that of Pin VAUXFBP. The internal power BJT is then
switched ON for a fixed–ON–time (or until the internal
current limit is reached), and coil current is allowed to build
up. As the BJT is switched OFF, coil current will flow
through the external Schottky diode to charge up the bulk
capacitor. After a fixed–mimimum–OFF time elapses, next
switching cycle will start if the output of the voltage
+ @ ǒ ) Ǔ ^{comparator is HIGH. Refer to Figure 23, the VAUX}
regulation level is determined by the equation as follows,
VAUX
VAUXFBP
1
RAUXb
RAUXa
(V)
Where Max ON Time, TON2, and Min OFF Time, TOFF2
can be determined by the following equations.
+ TON ₂ 1.7 10–11 RIref (S)
+ TOFF ₂ 2.1 10–12 RIref (S)
VBAT
RAUXa
200 kOhm
VAUXREF VAUXFBP
19
18
VAUXFBN
20
2.2 V
VAUXADJ
VAUXCON
VAUXEN
6–Bit
Counter
6–Bit
6
1.1 V
15 _Input
16 Logic
17
VCOMP
COMP1
RAUXb
2200 kOhm
+ve Edge Delay
for Max. ON Time
RQ
S Qb
L2
33uH
VBAT
VAUXBDV VAUXSW
21
25
senseBJT
Q1
+
CAUX
33 uF
VAUXEMR
26
1–SHOT
for Min. OFF Time
ILIM
COMP2
Auxiliary Level Control
Auxiliary Regulator
Figure 23. Simplified Block Diagram of Auxiliary Regulator
AGND
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