Datasheet

MAX17116
Dual-Output DC/DC
Power Supply for AMOLED
11
Step-Up Regulator
The step-up regulator is a constant-frequency current-
mode type. It operates at a 1.4MHz switching frequency
to allow the use of tiny 0.6mm thin inductors. The IC’s
internal digital soft-start, internal MOSFET switch, and
synchronous rectifier reduce the number of external
components for a very compact application circuit.
The regulator controls the output voltage and the power
delivered to the output by modulating duty cycle D of
the internal power MOSFET in each switching cycle. An
error amplifier compares the feedback signal with an
internal reference voltage and changes its output inter-
nal compensation node with its pole/zero series resistor
and capacitor to set the peak inductor current. As the
load varies, the error amplifier sources or sinks current
to the compensation node accordingly to produce the
inductor peak current necessary to service the load. To
maintain stability at high duty cycles, a slope compensa-
tion signal is added.
On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The
current through the inductor ramps up linearly, stor-
ing energy in its magnetic field. Once the sum of the
current-feedback signal and the slope compensation
exceed the compensation voltage, the controller resets
the flip-flop, turning off the MOSFET and turning on
the synchronous rectifier. Since the inductor current is
continuous, a transverse potential develops across the
inductor and the inductor sources current to the output.
The voltage across the inductor then becomes the differ-
ence between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.
The converter operates in skip mode only at very light
loads (typically it is less than 4mA) and includes a damp-
ing switch that turns on when the synchronous rectifier
turns off at zero or negative current to control LXP ringing.
Inverting Converter
The inverting converter is also a constant-frequency
(1.4MHz) current-mode type and includes synchro-
nous rectification to lower application of BOM cost and
improve efficiency. The inverter operates in skip mode
only at light loads and includes a damping switch that
turns on when the synchronous rectifier turns off at zero
or negative current. The switch prevents ringing in the
inductor in discontinuous conduction and the resulting
RF noise.
The inverting converter operates similarly to the step-up
converter, except that the main switch is a p-channel
MOSFET between LXN and IN. Energy is stored in the
inductor during the switch on-time and the continuous
inductor current pulls current from the output to ground
when the flip-flop resets, the main switch turns off, and
the synchronous rectifier turns on.
The internally set output voltage for the inverting con-
verter is adjustable between -5.4V and -1.5V in 100mV
steps. Adjustment is accomplished though a unique
control interface using the EN pin as shown in the EN
Serial Interface section. The step rate while program-
ming V
OUTN
is also adjustable through STEP pin set-
tings shown in the Electrical Characteristics table.
EN Serial Interface
The enable pin (EN) is used as an on/off pin, as well
as a serial interface input. When EN goes high, the IC
starts operation only after a 300Fs delay. Similarly, when
EN falls, the IC enters the shutdown state only after a
60Fs delay. This makes the EN pin available for serial-
data input as long as the pin state keeps changing fast
enough to avoid entering shutdown (12kHz or greater).
The interface protocol is a simple correspondence
between the number of pulses to ground on EN (1 to 40)
and the desired output voltage (-5.4V to -1.5V). Table 3
shows the relationship between the number of pulses
and the desired output voltage for the driver IC.