Low-Noise, 14V Input, 1A, PWM
3.3V AT 1A
cations. A SYNC input allows synchronization to an
external clock. The MAX1684/MAX1685 can operate in
five modes. Setting the devices to operate in the appro-
priate mode for the intended application (Table 1)
achieves highest efficiency.
The MAX1684/MAX1685 use an oscillator-triggered min-
imum/maximum on-time current-mode control scheme
(Figure 2). The minimum on-time is typically 220ns
unless the regulator is in dropout. The maximum on-time
is 2 / f
, allowing operation to 100% duty cycle.
Current-mode feedback provides cycle-by-cycle current
limiting for superior load- and line-transient response.
At each falling edge of the internal oscillator, the inter-
nal P-channel MOSFET (main switch) turns on. This
allows current to ramp up through the inductor to the
load and stores energy in a magnetic field. The switch
remains on until either the current-limit comparator
trips, the maximum on-time expires, or the PWM com-
parator signals that the output is in regulation. 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 out-
put diode 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 cur-
rent is low, smoothing the voltage across the load.
During normal operation, the MAX1684/MAX1685 regu-
late the output voltage by switching at a constant fre-
quency and modulating the power transferred to the
load on each cycle using the PWM comparator. A multi-
input comparator sums three weighted differential sig-
nals (the output voltage with respect to the reference,
the main switch current sense, and the slope-compen-
sation ramp) and changes states when a threshold is
reached. It modulates output power by adjusting the
USE 22µH FOR MAX1684
Figure 1. Standard Application Circuit
The MAX1684/MAX1685 step-down, pulse-width-modu-
lation (PWM) DC-DC converters provide an adjustable
output from 1.25V to the input voltage. They accept
inputs from 2.7V to 14V and deliver up to 1.6A. An inter-
nal MOSFET and synchronous rectifier reduce PC
board area while maintaining high efficiency. Operation
with up to 100% duty cycle minimizes dropout voltage.
Fixed-frequency PWM operation reduces interference
in sensitive communications and data-acquisition appli-
Table 1. Operating Modes
Fixed-Input Clock-Frequency PWM
PFM at light loads (<150mA). Fixed-
frequency PWM at heavy loads (>150mA).
Low-Power or Standby Mode.