Low-Noise, 5.5V-Input,
PWM Step-Down Regulator
Detailed Description
The MAX1692 step-down, pulse-width-modulated
(PWM), DC-DC converter has an adjustable output
range from 1.25V to the input voltage. An internal syn-
chronous rectifier improves efficiency and eliminates an
external Schottky diode. Fixed-frequency operation
enables easy post-filtering, thereby providing excellent
noise characteristics. As a result, the MAX1692 is an
ideal choice for many small wireless systems.
The MAX1692 accepts inputs as low as +2.7V while still
delivering 600mA. The MAX1692 can operate in four
modes to optimize performance. A forced (PWM) mode
switches at a fixed frequency, regardless of load, for
easy post-filtering. A synchronizable PWM mode uses
an external clock to minimize harmonics. A PWM/PFM
mode extends battery life by operating in PWM mode
under heavy loads and PFM mode under light loads for
reduced power consumption. Shutdown mode reduces
quiescent current to 0.1µA.
V
IN
+2.7V TO +5.5V
IN
C1
10µF
LIM
LX
C2
47µF
L1
10µH
V
OUT
= 1.8V @ 600mA
MAX1692
MAX1692
ON
/
OFF
C4
0.22µF
SHDN
REF
BP
C3
0.1µF
SYNC/
PWM
GND
PGND
C5
47pF
FB
R2
300k
R1
138k
PWM Control Scheme
The MAX1692 uses a slope-compensated, current-
mode PWM controller capable of achieving 100% duty
cycle. The device uses an oscillator-triggered, mini-
mum on-time, current-mode control scheme. The mini-
mum on-time is approximately 150ns unless in dropout.
The maximum on-time is approximately 2/f
OSC
, allow-
ing operation to 100% duty cycle. Current-mode feed-
back provides cycle-by-cycle current limiting for
superior load- and line-response and protection of the
internal MOSFET and rectifier.
At each falling edge of the internal oscillator, the SYNC
cell sends a PWM ON signal to the control and drive
logic, turning on the internal P-channel MOSFET (main
switch) (Figure 1). This allows current to ramp up
through the inductor (Figure 2) to the load, and stores
energy in a magnetic field. The switch remains on until
either the current-limit (LIM) comparator is tripped or
the PWM comparator signals that the output is in regu-
lation. When the switch turns off during the second half
of each cycle, the inductor’s magnetic field collapses,
releasing the stored energy and forcing current through
the N-channel synchronous rectifier to the output-filter
capacitor and load. The output-filter capacitor stores
charge when the inductor current is high and releases
it when the inductor current is low, thus smoothing the
voltage across the load.
During normal operation, the MAX1692 regulates out-
put voltage by switching at a constant frequency and
then modulating the power transferred to the load each
cycle using the PWM comparator. A multi-input com-
parator sums three weighted differential signals: the
Figure 2. Standard Application Circuit
output voltage with respect to the reference, the main
switch current sense, and the slope-compensation
ramp. It modulates output power by adjusting the
inductor-peak current during the first half of each cycle,
based on the output-error voltage. The MAX1692’s loop
gain is relatively low to enable the use of a small, low-
valued output-filter capacitor. The resulting load regula-
tion is 1.3% (typ) at 0 to 600mA.
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscilla-
tor cycle, which permits operation up to 100% duty
cycle. As the input voltage drops, the duty cycle
increases until the P-channel MOSFET is held on con-
tinuously. Dropout voltage in 100% duty cycle is the
output current multiplied by the on-resistance of the
internal switch and inductor, around 280mV (I
OUT
=
600mA). In PWM mode, subharmonic oscillation can
occur near dropout but subharmonic voltage ripple is
small, since the ripple current is low.
Synchronous Rectification
An N-channel, synchronous-rectifier improves efficien-
cy during the second half of each cycle (off time).
When the inductor current ramps below the threshold
set by the NEGLIM comparator (Figure 1) or when the
PWM reaches the end of the oscillator period, the syn-
chronous rectifier turns off. This keeps excess current
from flowing backward through the inductor, from the
output-filter capacitor to GND, or through the switch
and synchronous rectifier to GND. During PWM opera-
tion, the NEGLIM threshold adjusts to permit small
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