MAX767 Maxim, MAX767 Datasheet - Page 8

• Manufacturer Part Number: MAX767
• Description: 5V-to-3.3V / Synchronous / Step-Down Power-Supply Controller
• Manufacturer: Maxim

The main gain block is an open-loop comparator that
sums four signals: output voltage error signal, current-
sense signal, slope-compensation ramp, and the 3.3V
reference. This direct-summing method approaches
the ideal of cycle-by-cycle control of the output voltage.
Under heavy loads, the controller operates in full PWM
mode. Every pulse from the oscillator sets the output
latch and turns on the high-side switch for a period
determined by the duty factor (approximately
V
As the high-side switch turns off, the synchronous recti-
fier latch is set; 60ns later, the low-side switch turns on.
The low-side switch stays on until the beginning of the
next clock cycle (in continuous-conduction mode) or
until the inductor current reaches zero (in discontinu-
ous-conduction mode). Under fault conditions where
the inductor current exceeds the 100mV current-limit
threshold, the high-side latch resets and the high-side
switch turns off.
At light loads, the inductor current fails to exceed the
25mV threshold set by the minimum-current compara-
tor. When this occurs, the PWM goes into Idle-Mode™,
skipping most of the oscillator pulses to reduce the
switching frequency and cut back switching losses.
The oscillator is effectively gated off at light loads
5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller
Figure 2. Kelvin Connections for the Current-Sense Resistor
™ Idle-Mode is a trademark of Maxim Integrated Products.
8
OUT
_______________________________________________________________________________________
/ V
IN
).
FAT, HIGH-CURRENT TRACES
MAIN CURRENT PATH
MAX767
SENSE RESISTOR
because the minimum-current comparator immediately
resets the high-side latch at the beginning of each
cycle, unless the FB signal falls below the reference
voltage level.
Connecting a capacitor from the soft-start pin (SS) to
ground allows a gradual build-up of the 3.3V output
after power is applied or ON is driven high. When ON is
low, the soft-start capacitor is discharged to GND.
When ON is driven high, a 4µA constant current source
charges the capacitor up to 4V. The resulting ramp volt-
age on SS linearly increases the current-limit compara-
tor set-point, increasing the duty cycle to the external
power MOSFETs. With no soft-start capacitor, the full
output current is available within 10µs (see Applications
Information and Design Procedure section).
Synchronous rectification allows for high efficiency by
reducing the losses associated with the Schottky rectifi-
er. Also, the synchronous-rectifier MOSFET is neces-
sary for correct operation of the MAX767’s boost gate-
drive supply.
When the external power MOSFET (N1) turns off, ener-
gy stored in the inductor causes its terminal voltage to
reverse instantly. Current flows in the loop formed by
the inductor (L1), Schottky diode (D2), and the load—
an action that charges up the output filter capacitor
(C2). The Schottky diode has a forward voltage of
about 0.5V which, although small, represents a signifi-
cant power loss and degrades efficiency. The synchro-
nous-rectifier MOSFET parallels the diode and is turned
on by DL shortly after the diode conducts. Since the
synchronous rectifier’s on resistance (r
low, the losses are reduced. The synchronous-rectifier
MOSFET is turned off when the inductor current falls to
zero.
The MAX767’s internal break-before-make timing
ensures that shoot-through (both external switches
turned on at the same time) does not occur. The
Schottky rectifier conducts during the time that neither
MOSFET is on, which improves efficiency by preventing
the synchronous-rectifier MOSFET’s lossy body diode
from conducting.
The synchronous rectifier works under all operating
conditions, including discontinuous-conduction mode
and idle-mode.
Synchronous Rectifier
DS(ON)
Soft-Start
) is very