5 V/3 .3 V o r Ad ju s t a b le , 1 0 0 % Du t y-Cyc le ,
Hig h -Effic ie n c y, S t e p -Do w n DC-DC Co n t ro lle rs
/MAX1627
turns off. Otherwise the MOSFET remains on, allowing a
_______________De t a ile d De s c rip t io n
duty cycle of up to 100%. This feature ensures the low-
The MAX1626/MAX1627 are step-down DC-DC con-
est possible dropout. Once the MOSFET is turned off,
trollers designed primarily for use in portable comput-
the minimum off-time comparator keeps it off. The mini-
ers and battery-powered devices. Using an external
mum off-time is normally 2µs, except when the output is
MOSFET and current-sense resistor allows design flexi-
significantly out of regulation. If the output is low by
bility and the improved efficiencies associated with
30% or more, the minimum off-time increases, allowing
high-performance P-channel MOSFETs. A unique, cur-
soft-start. The error comparator has 0.5% hysteresis for
rent-limited, pulse-frequency-modulated (PFM) control
improved noise immunity.
scheme gives these devices excellent efficiency over
In the MAX1626, the 3/5 pin selects the output voltage
(Figure 2). In the MAX1627, external feedback resistors
at FB adjust the output.
load ranges up to three decades, while drawing around
90µA under no load. This wide dynamic range opti-
mize s the MAX1626/MAX1627 for b a tte ry-p owe re d
applications, where load currents can vary consider-
ably as individual circuit blocks are turned on and off to
c ons e rve e ne rg y. Op e ra tion to a 100% d uty c yc le
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.
Op e ra t in g Mo d e s
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.
P FM Co n t ro l S c h e m e
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 delivering high output currents, the MAX1626/
MAX1627 op e ra te in c ontinuous -c ond uc tion mod e
(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.
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
a nd 2). As s hown in Fig ure 3, c urre nt throug h 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.
1 0 0 % Du t y Cyc le a n d Dro p o u t
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:
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
Dropout Voltage =
I
x R
+ R
+ R
SENSE INDUCTOR
OUT
[
DS(ON)
]
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
