MAX8893A Datasheet, PDF(28/50 Page) Maxim Integrated Products – μPMICs for Multimedia Application Processors in a 3.0mm x 2.5mm WLP

◁

ÂµPMICs for Multimedia Application Processors

in a 3.0mm x 2.5mm WLP

Detailed Description

The MAX8893A/MAX8893B/MAX8893C highly integrat-

ed power-management ICs integrate a high-efficiency

500mA step-down DC-DC converter, five low-dropout

linear regulators, a load switch with ultra-low on-resis-

tance, a USB high-speed switch, and a 400kHz I2C

serial interface.

The step-down converter delivers over 500mA at I2C

programmable output levels from 0.8V to 2.4V. It uses a

proprietary hysteretic-PWM control scheme that switch-

es up to 4MHz, allowing a trade-off between efficiency

and tiny external components. The step-down converter

also features dynamic voltage scaling (DVS) control. Its

output voltage ramps up with the I2C-controlled ramp

rate from 1mV/Fs to 12mV/Fs.

Five low-dropout linear regulators feature low 45FVRMS

output noise (LDO1, LDO4, and LDO5) and very low

ground currents (LDO2 and LDO3).

The USB high-speed switch is a high ESD-protected

DPDT analog switch. It is ideal for USB 2.0 Hi-Speed

(480Mbps) switching applications and also meets USB

low- and full-speed requirements. The load switch fea-

tures ultra-low on-resistance and operates from 0.8V

to 2.4V input range. Its rise time is I2C programmable

to control the inrush current. The internal I2C interface

provides flexible control on regulator ON/OFF control,

output voltage setting, step-down dynamic voltage scal-

ing and ramp rate, and load switch timing.

Step-Down DC-DC Converter

Control Scheme

The MAX8893A/MAX8893B/MAX8893C step-down con-

verter is optimized for high-efficiency voltage conver-

sion over a wide load range, while maintaining excellent

transient response, minimizing external component size,

and output voltage ripple. The step-down converter also

features an optimized on-resistance internal MOSFET

switch and synchronous rectifier to maximize efficiency.

The IC utilizes a proprietary hysteretic-PWM control

scheme that switches with nearly fixed frequency up to

4MHz allowing for ultra-small external components. Its

output current is guaranteed up to 500mA.

When the step-down output voltage falls below the regu-

lation threshold, the error comparator begins a switching

cycle by turning on the high-side switch. This switch

remains on until the minimum on-time (tON) expires and

the output voltage is in regulation or the current-limit

threshold is exceeded. Once off, the high-side switch

remains off until the minimum off-time (tOFF) expires

and the output voltage again falls below the regulation

threshold. During the off period, the low-side synchro-

nous rectifier turns on and remains on until either the

high-side switch turns on again or the inductor current

reduces to the rectifier-off current threshold (ILXOFF =

30mA (typ)). The internal synchronous rectifier elimi-

nates the need for an external Schottky diode.

The step-down converter has the internal soft-start cir-

cuitry with a fixed ramp to eliminate input current spikes

when it is enabled.

Voltage Positioning Load Regulation

The step-down converter uses a unique feedback net-

work. By taking feedback from the LX node, the usual

phase lag due to the output capacitor is removed, mak-

ing the loop exceedingly stable and allowing the use of

very small ceramic output capacitors. This configuration

causes the output voltage to shift by the inductor series

resistance multiplied by the load current. This voltage-

positioning load regulation greatly reduces overshoot

during load transients, which effectively halves the

peak-to-peak output-voltage excursions compared to

traditional step-down converters.

Dynamic Voltage Scaling (DVS)

Control with Ramp Rate

The step-down output voltage has a variable ramp rate

that is set by the BUCKRAMP bits in the DVS RAMP

CONTROL register. This register controls the output-

voltage ramp rate during a positive voltage change (for

example, from 1.0V to 1.1V), and a negative voltage

change (for example, from 1.1V to 1.0V). Ramp rate

adjustment range is from 1mV/Fs to 12mV/Fs in the step

of 1mV/Fs.

After the step-down converter is in regulation, its output

voltage can dynamically ramp up at the rate set by the

BUCKRAMP bits for a positive voltage change. For a

negative voltage change, the decay rate of the output

voltage depends on the size of the external load: a small

load results in an output-voltage decay that is slower

than the specified ramp rate and LX sinks current from

the output capacitor to actively ramp down the output

voltage; a large load (greater than COUT x Ramp Rate)

results in an output-voltage decay with the specified

ramp rate.

When the step-down converter is disabled, the output

voltage decays to ground at a rate determined by the

output capacitance, internal discharge resistance, and

the external load.

▷