MC34280 Datasheet, PDF (10/20 Pages) ON Semiconductor – Power Supply & Management IC for Handheld Electronic Products

MC34280
DETAILED OPERATING DESCRIPTION
General
The MC34280 is a power supply integrated circuit which
provides two boost regulated outputs and some power
management supervisory functions. Both regulators apply
Pulse–Frequency–Modulation (PFM). The main boost
regulator output can be externally adjusted from 2.7V to 5V.
An internal synchronous rectifier is used to ensure high
efficiency (achieve 87%). The auxiliary regulator with a
built–in power transistor can be configured to produce a
wide range of positive voltage (can be used to supply a LCD
contrast voltage). This voltage can be adjusted from +5V to
+25V by an external potentiometer; or by a microprocessor,
digitally through a 6–bit internal DAC.
The MC34280 has been designed for battery powered
hand–held products. With the low start–up voltage from 1V
and the low quiescent current (typical 35 µA); the MC34280
is best suited to operate from 1 to 2 AA/ AAA cell.
Moreover, supervisory functions such as low battery
detection, CPU power–on reset, and back–up battery
control, are also included in the chip. It makes the MC34280
the best one–chip power management solution for
applications such as electronic organizers and PDAs.
Pulse Frequency Modulation (PFM)
Both regulators apply PFM. With this switching scheme,
every cycle is started as the feedback voltage is lower than
the internal reference. This is normally performed by
internal comparator. As cycle starts, Low–Side switch (i.e.
M1 in Figure 1) is turned ON for a fixed ON time duration
(namely, Ton) unless current limit comparator senses coil
current reaches its preset limit. In the latter case, M1 is OFF
instantly. So Ton is defined as the maximum ON time of M1.
When M1 is ON, coil current ramps up so energy is being
stored inside the coil. At the moment just after M1 is OFF,
the Synchronous Rectifier (i.e. M2 in Figure 1) or any
rectification device (such as Schottky Diode of Auxiliary
Regulator) is turned ON to direct coil current to charge up
the output bulk capacitor. Provided that coil current is not
reached, every switching cycle delivers fixed amount of
energy to the bulk capacitor. So for higher loading, larger
amount of energy (Charge) is withdrawn from the bulk
capacitor, and as output voltage is needed to regulated, larger
amount of Charge is needed to be supplied to the bulk
capacitor, that means switching frequency is needed to be
increased; and vice–versa.
Main Regulator
Figure 20 shows the simplified block diagram of Main
Regulator. Notice that precise bias current Iref is generated
by a VI converter and external resistor RIref, where
+ Iref
0.5
RIref
(A)
This bias current is used for all internal current bias as well
as setting VMAIN value. For the latter application, Iref is
doubled and fed as current sink at Pin 1. With external
resistor RMAINb tied from Pin1 to Pin32, a constant level
shift is generated in between the two pins. In close–loop
operation, voltage at Pin 1 (i.e. Output feedback voltage) is
needed to be regulated at the internal reference voltage level,
1.22V. Therefore, the delta voltage across Pin 1 and Pin 32
which can be adjusted by RMAINb determines the Main
Output voltage. If the feedback voltage drops below 1.22V,
internal comparator sets switching cycle to start. So, VMAIN
can be calculated as follows.
+ ) VMAIN
1.22
RMAINb
RIref
(V)
From the above equation, although VMAIN can be
adjusted by RMAINb and RIref ratio, for setting VMAIN, it
is suggested, by changing RMAINb value with RIref kept at
480K. Since changing RIref will alter internal bias current
which will affect timing functions of Max ON time (TON1)
and Min OFF time (TOFF1). Their relationships are as
follows;
+ TON ₁ 1.7 10–11 RIref (S)
+ TOFF ₁ 6.4 10–12 RIref (S)
Continuous Conduction Mode and Discontinuous
Conduction Mode
In Figure 21, regulator is operating at Continuous
Conduction Mode. A switching cycle is started as the output
feedback voltage drops below internal voltage reference
VREF. At that instant, the coil current does not drop to zero
yet, and it starts to ramp up for the next cycle. As the coil
current ramps up, loading makes the output voltage to
decrease as the energy supply path to the output bulk
capacitor is disconnected. And after Ton elapsed, M1 is OFF,
M2 becomes ON, energy is dumped to the bulk capacitor.
Output voltage is increased as excessive charge is pumped
in, then it is decreased after the coil current drops below the
loading. Notice the abrupt spike of output voltage is due to
ESR of the bulk capacitor. Feedback voltage can be
resistor–divided down or level–shift down from the output
voltage. As this feedback voltage drops below VREF, next
switching cycle starts.
http://onsemi.com
10