MAX1927/MAX1928
Detailed Description
The MAX1927/MAX1928 PWM step-down DC-DC con-
verters accept inputs as low as 2.6V, while delivering
800mA to output voltages as low as 0.75V. These
devices operate in one of two modes to optimize noise
and quiescent current. Under heavy loads, MAX1927/
MAX1928 operate in pulse-width modulation (PWM)
mode and switch at a fixed 1MHz frequency. Under
light loads, they operate in PFM mode to reduce power
consumption. In addition, both devices provide selec-
table forced PWM operation for minimum noise at all
load currents.
PFM Operation and PWM Control Scheme
The PFM mode improves efficiency and reduces quies-
cent current to 140µA at light loads. The MAX1927/
MAX1928 initiate pulse-skipping PFM operation when
the peak inductor current drops below 130mA. During
PFM operation, the MAX1927/MAX1928 switch only as
necessary to service the load, reducing the switching
frequency and associated losses in the internal switch,
synchronous rectifier, and inductor.
During PFM mode, a switching cycle initiates when the
error amplifier senses that the output voltage has
dropped below the regulation point. If the output volt-
age is low, the P-channel MOSFET switch turns on and
conducts current to the output filter capacitor and load.
The PMOS switch turns off when the PWM comparator
is satisfied. The MAX1927/MAX1928 then wait until the
error amplifier senses a low output voltage to start
again. Some jitter is normal during the transition from
PFM to PWM with loads around 100mA. This has no
adverse impact on regulation.
At loads greater than 130mA, the MAX1927/MAX1928
use a fixed-frequency, current-mode, PWM controller
capable of achieving 100% duty cycle. Current-mode
feedback provides cycle-by-cycle current limiting,
superior load and line response, as well as overcurrent
protection for the internal MOSFET and synchronous
rectifier. A comparator at the P-channel MOSFET switch
detects overcurrent conditions exceeding 1.1A.
During PWM operation, the MAX1927/MAX1928 regu-
late output voltage by switching at a constant frequency
and then modulating the power transferred to the load
using the PWM comparator (Figure 1). The error-amp
output, the main switch current-sense signal, and the
slope compensation ramp are all summed at the PWM
comparator. The comparator modulates the output
power by adjusting the peak inductor current during the
first half of each cycle based on the output-error volt-
age. The MAX1927/MAX1928 have relatively low AC-
loop gain coupled with a high-gain integrator to enable
the use of a small, low-valued, output filter capacitor.
The resulting load regulation is 0.3% (typ) from 0 to
800mA.
Forced PWM Operation
To force PWM-only operation, connect PWM to BATT.
Forced PWM operation is desirable in sensitive RF and
data-acquisition applications to ensure that switching
noise does not interfere with sensitive IF and data sam-
pling frequencies. A minimum load is not required dur-
ing forced PWM operation because the synchronous
rectifier passes reverse inductor current as needed to
allow constant frequency operation with no load.
Forced PWM operation has higher quiescent current
than PFM (2mA typ compared to 140µA) due to contin-
uous switching.
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscilla-
tor cycle, which permits operation at 100% duty cycle.
As the input voltage drops, the duty cycle increases
until the internal P-channel MOSFET stays on continu-
ously. Dropout voltage at 100% duty cycle is the output
current multiplied by the sum of the internal PMOS on-
resistance (typically 0.25Ω) and the inductor resis-
tance. Near dropout, switching cycles can be skipped,
reducing switching frequency. However, voltage ripple
remains small because the current ripple is still low.
Synchronous Rectification
An N-channel synchronous rectifier eliminates the need
for an external Schottky diode and improves efficiency.
The synchronous rectifier turns on during the second
half of each cycle (off-time). During this time, the volt-
age across the inductor is reversed, and the inductor
current falls. In normal mode, the synchronous rectifier
is turned off when either the output falls out of regula-
tion (and another on-time begins) or when the inductor
current approaches zero. In forced PWM mode, the
synchronous rectifier remains active until the beginning
of a new cycle.
Shutdown Mode
Driving SHDN to GND places the MAX1927/MAX1928
in shutdown mode. In shutdown, the reference, control
circuitry, internal switching MOSFET, and synchronous
rectifier turn off and the output becomes high imped-
ance. Drive SHDN high for normal operation. Input cur-
rent falls to 0.1µA (typ) during shutdown mode.
POK Output
POK is an open-drain output that goes high impedance
20ms after the soft-start ramp has concluded and V
FB
is within 90% of the threshold. POK is low impedance
when in shutdown.
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
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