Power supply and management ICs for handheld electronic devices

Motorola's MC34280 power integrated circuit provides two boost regulated outputs and power management monitoring functions. Both regulators use pulse frequency modulation (PFM), and the main boost regulator output is externally adjustable from 2.7 to 5V. Internal synchronous rectification ensures high efficiency (up to 87%). The auxiliary voltage regulator can be configured to provide a wide range of positive voltage (can be used for LCD contrast voltage), this voltage can be adjusted by an external potentiometer or microprocessor, and the adjustment range is from +5V to +25V.

The MC34280 is designed for battery-powered handheld devices. It has low startup voltage (1V) and low quiescent current (35μA), and is particularly suitable for working with 1 to 2 AA/AAA batteries. Monitoring functions include: low battery detection, CPU power-on reset and backup battery control. Applications include: digital compilers and digital dictionaries, digital assistants (PDAs), dual output power supplies (for MPU, Logic, Memory, LCD) and handheld battery-powered devices (1 to 2 AA/AAA batteries).

pulse frequency modulation

Both MC34280 voltage regulators use PFM. In this switching method, each cycle begins when the feedback voltage is lower than the internal reference voltage. This is usually performed by an internal comparator. As the cycle begins, the low-side switch (M1 in Figure 1) conducts (for a fixed on-time Ton) unless the current limit comparator sense coil current reaches its preset limit. When the latter situation occurs, M1 is shut down immediately. So Ton is defined as the maximum conduction time of M1. When M1 is turned on, the coil current rises straight up, storing energy in the coil. Just after M1 turns off, the synchronous rectifier (such as the Schottky diode of the auxiliary regulator) conducts allowing the coil current to charge the output capacitor. When the provided coil current is not reached, each switching cycle delivers a fixed amount of energy to the capacitor. So the higher the load, the greater the energy (charge) taken out of the capacitor, and just as the output voltage needs to be stable, a larger amount of charge is required to supply the capacitor, which means that the switching frequency needs to be increased, and vice versa.

main regulator

Figure 2 shows a simplified block diagram of the main regulator. The precision bias current Iref is generated by the V1 converter and the external resistor Rlref:

Iref=0.5 / RIref (A)

The bias current is used to bias the internal current and set the VMAIN value. For the latter application, Iref is doubled and fed into pin 1 as a current source. Connect an external resistor RMAINb between Pin 1 and Pin 32 to create a constant level offset between the two pins. When operating in closed loop, the voltage on pin 1 (i.e. the output feedback voltage) needs to be adjusted to the internal reference voltage level of 1.22V. So the delta voltage across pins 1 and 32 (adjustable with RMAINb) determines the main output voltage. If the feedback voltage decreases by 1.22V, the internal comparator is set to start the switching cycle. Therefore, VMAIN can be calculated by the following formula

VMAIN1.22+RMAINb/RIref (V)

From the above equation, it can be seen that although VMAIN can be adjusted by the ratio of RMAINb and RIref, it is recommended that RIref be kept at 480K. Changing the RMAINb value will change the internal bias current, which will affect the maximum on-time (TON1) and minimum off-time (TOFF1) timing functions. The relationship between TON1TOFF1 and RIref is:

TON1=1.7×10-11×RIref (S)

TOFF1=6.4×10-12×RIref (S)

Auxiliary regulator

The auxiliary regulator is a boost regulator that uses PFM methods to increase efficiency and reduce quiescent current. An internal voltage comparator (COMP1 in Figure 3) detects when the voltage on pin VAUXFBN drops below the voltage on pin VAUXBP. The internal power BJT then turns on for a fixed on-time (or until the internal current limit is reached) and allows the coil current to build up. As the BJT turns off, coil current will flow through the external Schottky diode and charge the output capacitor. After a fixed minimum off-time, if the voltage comparator output is high, the next switching cycle begins. The VAUX regulation level is determined by the following equation:

VAUX=VAUXFBP·(1+RAUXb / RAUXa) (V)

The maximum on-time TON2 and the minimum off-time TOFF2 are:

TON2=1.7×10-11×RIref (S)

TOFF2=6.4×10-12×RIref (S)

Current limit for both regulators

In Figures 2 and 3, the sensing device (sense FET or sense BJT) samples the coil current when the low-side switch is on. The sampling current flows through the sensing resistor, producing a sensing voltage. The threshold detector (COMP2 in Figures 2 and 3) detects whether the sense voltage is above a preset level. If a preset level is exceeded, the detector output resets the flip-flop, turns off the low-side switch, and turns on the switch only at the beginning of the next cycle.

The following explains the management functions of MC3480

Low battery detection

The low battery detection unit is actually a voltage comparator. If the voltage of the external pin LOWBATSEN is lower than the 0.85V internal reference voltage, the pin LOWBAT is in a low state. If the voltage of the external pin LOWBATSEN returns to greater than 1.1V, the LOWBAT becomes a high state. As can be seen in Figure 1, using external resistors BLBa and RLBb, the low battery detection threshold can be adjusted according to the following equation:

VLOBAThigh=1.1×(1+RLBa / RLBb) (V)

VLOBATlow=0.85×(1+RLBa / RLBb) (V)

Power on reset

After the battery is inserted, the power-on reset unit accepts an external valid high-state enable signal to stimulate the MC38280. During startup, the internal startup circuit is enabled to boost VAMIN to a voltage level that is compensated by the user-determined VMAIN output level minus 0.15V. The internal power-on reset signal then disables excitation of the main and auxiliary regulators. At the same time, the starting circuit is closed. The power-on reset unit also starts charging the external capacitor connected between pin PDELAY and ground with a precise constant current. When the PDELAY pin voltage reaches the internally set threshold, the PORB pin will turn high to wake up the microprocessor.

Backup battery control

The backup conduction path provided by the internal power switch can be controlled by internal logic or a microprocessor. If LIBATCI is low, the switch controlled by internal logic turns on when the battery is removed and VMAIN drops more than 100mV below LIBATIM, and returns to the off state when the battery is inserted. If LIBATCL is high, the switch is controlled by the microprocessor via LIBATON.