Datasheet

Evaluates: MAX8655/MAX8688
Power-OK (POK)
POK is an open-drain output on the MAX8655 that mon-
itors the output voltage. When the output is above 92%
of its nominal regulation voltage, POK is high imped-
ance. When the output drops below 89% of its nominal
regulation voltage, POK is internally pulled low. POK is
also internally pulled low when the MAX8655 is shut
down or in a fault condition.
Soft-Start and REFIN
The internal soft-start circuitry gradually ramps up the
reference voltage to control the rate of rise of the output
voltage and reduce input surge currents during startup.
The soft-start period is determined by the value of the
capacitor from SS to GND. The soft-start time is approx-
imately given by:
where t
SS
= soft-start time in seconds and C
SS
is the
value of the capacitor at the SS pin.
The MAX8655 also features monotonic output-voltage
rise, which allows the MAX8655 to safely start up into a
prebiased output without pulling the output voltage down.
The MAX8655 has a reference input (REFIN). When an
external reference up to 1.5V is connected to REFIN,
the feedback regulation voltage is equal to the voltage
applied to REFIN.
Connect REFIN to AVL to use the internal 0.7V refer-
ence (jumper JP5).
Current Limit
The MAX8655 uses both foldback and peak current
limiting.
Peak Current Limit
The peak current-limit threshold (V
TH
) is set by a resis-
tor connected from ILIM1 to GND (R
ILIM1
). V
TH
corre-
sponds to the peak voltage across the sensing element
(inductor or current-sense resistor). R
ILIM1
is calculated
as follows:
The peak current limit is used to sense the inductor cur-
rent, and is more accurate than the valley current limit
since it does not depend upon the on-resistance of the
low-side MOSFET.
Valley Current Limit
The MAX8655 has an adjustable valley current limit,
configurable for foldback with automatic recovery, or
constant current limit with latch-up. The valley current is
sensed across the on-resistance of the low-side
MOSFET. When the latch-off mode is used, connect
MODE to AVL and set the current-limit threshold with
one resistor from ILIM2 to GND. To use foldback cur-
rent limit with auto recovery, connect MODE to GND
and connect a resistor from ILIM2 to the output (R11),
and another resistor from ILIM2 to GND (R16). Cycle
EN or input power to reset the current-limit latch. For
further information refer to the
Setting the Current Limit
section in the MAX8655 IC data sheet.
Overvoltage Protection (OVP)
The MAX8655 provides output overvoltage protection
(OVP), which is set independent of the output regula-
tion voltage with a resistor voltage-divider connected
between the output and GND (R9 and R14 of
Figure 1a). When the voltage at OVP exceeds the OVP
threshold, the regulator stops switching and latches on
the low-side power MOSFET. Cycle EN or the power
applied to AVL to clear the latch.
Select R14 between 5kΩ and 24kΩ, then calculate R9
with the following equation:
where V
OVP
= 1.15 x V
FB
and V
OUT
= voltage at which
OVP protection should set in.
Evaluating the MAX8688
The MAX8688 is a fully digital power-supply manager
that can be tested in conjunction with the MAX8655.
Samples of the MAX8688 IC can be ordered from
Maxim. To test the MAX8688, populate it with its circuit-
ry, as shown in the schematic or in the component list.
Populate R215 with 10kΩ to connect pin 3 of JP201
(RST) and pin 1 of JP204 (3.3V). Also, remove the shunt
on pins 1-2 of JP5. For further information, refer to the
MAX8688 IC data sheet.
RR
V
V
OUT
OVP
914 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
R
V
µA
ILIM
TH
1
75
10
=
×.
t
ms µF
C
SS SS
=
()
×
30 4./
MAX8655 Evaluation Kit
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