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MAX8855 Evaluation Kit
Detailed Description
The MAX8855 EV kit evaluates the MAX8855 dual step-
down regulator. The regulator switching frequency is
set to 1MHz by default and can be adjusted from
0.5MHz to 2MHz. The EV kit is designed to operate
from a DC power supply that can provide 2.35V to 3.6V
and 7A of current.
Regulator 1 (OUT1) is configured as a step-down regu-
lator set to 1.8V and can provide 5A of current.
Regulator 2 (OUT2) is configured as a step-down regu-
lator set to 1.2V and can provide 5A of current.
The EV kit output voltages are adjustable from a mini-
mum of 0.6V to a maximum of 0.9 x V IN . Refer to the
Setting the Output Voltage section of the MAX8855 IC
data sheet to set other output voltages in this range for
V OUT1 and V OUT2 .
Enable (EN1 and EN2)
The EV kit provides separate enable inputs, EN1 and
EN2, to individually control or sequence the output volt-
ages. The enable signals EN1 and EN2 are pulled up to
V DD by resistors R3 and R18, respectively. To enable
OUT1, remove the shunt from JU1; to disable OUT1,
place the shunt on JU1 pins 2-3. To enable OUT2,
remove the shunt from JU2; to disable OUT2, place the
shunt on JU2 pins 1-2.
The enable inputs can also be driven from an external
logic signal. With the JU1 and JU2 shunts removed,
connect the EN1 signal to pin 2 of JU1, and the EN2
signal to pin 2 of JU2. The device enable pins are
active-high and TTL compatible.
Power-Good Outputs (PWRGD_)
Regulator 1 and 2 provide power-good output signals
that indicate when the corresponding output is in regu-
lation. PWRGD1 and PWRGD2 are open-drain outputs
that are pulled up to V DD by a 20k Ω resistor. The
power-good, open-drain output for regulator 1
(PWRGD1) is high impedance when V REFIN ≥ 0.54V
and V FB1 ≥ 0.9 x V REFIN . PWRGD1 is low when V REFIN
< 0.54V, EN1 is low, V DD or V IN1 is below V UVLO , the
thermal-overload protection is activated, or when V FB1
< 0.9 x V REFIN .
PWRGD2 is high impedance when V SS2 ≥ 0.54V and
V FB2 ≥ 0.9 x V SS2 . PWRGD2 is low when V SS2 < 0.54V,
EN2 is low, V DD or V IN2 is below V UVLO , the thermal-over-
load protection is activated, or when V FB2 < 0.9 x V SS2 .
External Reference Input (REFIN)
The EV kit provides an option for an external reference
input. To use an external reference, remove the shunt
from JU3 and connect an external reference between
0V and V DD - 1.6V to the REFIN pad to set the FB1 reg-
ulation voltage. To use the internal 0.6V reference,
place a shunt on JU3 pins 1-2 to connect REFIN to SS1
(see Table 3). When the IC is shut down, REFIN is
pulled to GND through an internal 335 Ω .
If additional filtering is required, a 0.22μF capacitor can
be added (C5).
Tracking
The EV kit can be configured as a tracking circuit (com-
monly used in DDR supply applications). To configure
the EV kit for tracking, install resistors R1 and R19.
These resistors form a voltage-divider that sets the
tracking ratio. Remove R6 so OUT1 regulates to the
REFIN voltage. Set jumper JU3 to pins 2-3. Refer to the
MAX8855 IC data sheet for more information on track-
ing circuits and component selection.
Sequencing
To enable sequencing on the EV kit, place a shunt on
pins 1-2 of JU1. In this configuration, OUT1 remains
disabled until OUT2 is enabled and reaches regulation.
When OUT1 is disabled, OUT2 shuts down at the same
time. Refer to the MAX8855 IC data sheet for more
information on sequencing.
Switching Frequency and
Synchronization (FSYNC)
The EV kit operates from an internal 0.5MHz to 2MHz
switching frequency set by R5. The EV kit’s switching
frequency is set to 1MHz by default. To change the
switching frequency, replace R5 with a resistor value
calculated as shown in the Setting the Switching
Frequency section of the MAX8855 IC data sheet.
The EV kit can also be synchronized to an external
clock from 250kHz to 2.5MHz by connecting the clock
to the FSYNC BNC input. The external clock must have
a duty cycle between 10% and 90% to ensure the regu-
lators operate 180 ° out-of-phase.
Thermal-Overload Protection
Thermal-overload protection limits the total power dissi-
pation of the device. Internal thermal sensors monitor
the internal die temperature of each regulator. When this
temperature exceeds +165 ° C, the corresponding regu-
lator is shut down, allowing the IC to cool. The thermal
sensor turns the regulator on after the junction tempera-
ture cools by 20 ° C. In a continuous thermal-overload
condition, this results in a pulsed output.
Current Limit
The EV kit provides both peak and valley current limits
to achieve robust short-circuit protection.
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