5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________Detailed Description
The MAX1626/MAX1627 are step-down DC-DC con-
trollers designed primarily for use in portable comput-
ers and battery-powered devices. Using an external
MOSFET and current-sense resistor allows design flexi-
bility and the improved efficiencies associated with
high-performance P-channel MOSFETs. A unique, cur-
rent-limited, pulse-frequency-modulated (PFM) control
scheme gives these devices excellent efficiency over
load ranges up to three decades, while drawing around
90µA under no load. This wide dynamic range opti-
mizes the MAX1626/MAX1627 for battery-powered
applications, where load currents can vary consider-
ably as individual circuit blocks are turned on and off to
conserve energy. Operation to a 100% duty cycle
allows the lowest possible dropout voltage, extending
battery life. High switching frequencies and a simple
circuit topology minimize PC board area and compo-
nent costs. Figure 1 shows a typical operating circuit
for the MAX1626.
turns off. Otherwise the MOSFET remains on, allowing a
duty cycle of up to 100%. This feature ensures the low-
est possible dropout. Once the MOSFET is turned off,
the minimum off-time comparator keeps it off. The mini-
mum off-time is normally 2µs, except when the output is
significantly out of regulation. If the output is low by
30% or more, the minimum off-time increases, allowing
soft-start. The error comparator has 0.5% hysteresis for
improved noise immunity.
In the MAX1626, the
3/5
pin selects the output voltage
(Figure 2). In the MAX1627, external feedback resistors
at FB adjust the output.
MAX1626/MAX1627
Operating Modes
When delivering low and medium output currents, the
MAX1626/MAX1627 operate in discontinuous-conduc-
tion mode. Current through the inductor starts at zero,
rises as high as the peak current limit set by the cur-
rent- sense resistor, then ramps down to zero during
each cycle (Figure 3). Although efficiency is still excel-
lent, output ripple increases and the switch waveform
exhibits ringing. This ringing occurs at the resonant fre-
quency of the inductor and stray capacitance, due to
residual energy trapped in the core when the commuta-
tion diode (D1 in Figure 1) turns off. It is normal and
poses no operational problems.
When delivering high output currents, the MAX1626/
MAX1627 operate in continuous-conduction mode
(Figure 4). In this mode, current always flows through
the inductor and never ramps to zero. The control cir-
cuit adjusts the switch duty cycle to maintain regulation
without exceeding the peak switching current set by
the current-sense resistor. This provides reduced out-
put ripple and high efficiency.
PFM Control Scheme
The MAX1626/MAX1627 use a proprietary, third-genera-
tion, current-limited PFM control scheme. Improvements
include a reduced current-sense threshold and operation
to a 100% duty cycle. These devices pulse only as need-
ed to maintain regulation, resulting in a variable switching
frequency that increases with the load. This eliminates the
current drain associated with constant-frequency pulse-
width-modulation (PWM) controllers, caused by switching
the MOSFET unnecessarily.
When the output voltage is too low, the error compara-
tor sets a flip-flop, which turns on the external P-chan-
nel MOSFET and begins a switching cycle (Figures 1
and 2). As shown in Figure 3, current through the
inductor ramps up linearly, storing energy in a magnet-
ic field while dumping charge into an output capacitor
and servicing the load. When the MOSFET is turned off,
the magnetic field collapses, diode D1 turns on, and
the current through the inductor ramps back down,
transferring the stored energy to the output capacitor
and load. The output capacitor stores energy when the
inductor current is high and releases it when the induc-
tor current is low.
The MAX1626/MAX1627 use a unique feedback and
control system to govern each pulse. When the output
voltage is too low, the error comparator sets a flip-flop,
which turns on the external P-channel MOSFET. The
MOSFET turns off when the current-sense threshold is
exceeded or when the output voltage is in regulation. A
one-shot enforces a 2µs minimum on-time, except in
current limit. The flip-flop resets when the MOSFET
100% Duty Cycle and Dropout
The MAX1626/MAX1627 operate with a duty cycle up
to 100%. This feature extends usable battery life by
turning the MOSFET on continuously when the supply
voltage approaches the output voltage. This services
the load when conventional switching regulators with
less than 100% duty cycle would fail. Dropout voltage
is defined as the difference between the input and out-
put voltages when the input is low enough for the out-
put to drop out of regulation. Dropout depends on the
MOSFET drain-to-source on-resistance, current-sense
resistor, and inductor series resistance, and is propor-
tional to the load current:
Dropout Voltage =
I
OUT
x R
DS(ON)
+ R
SENSE
+ R
INDUCTOR
[
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