Datasheet

Maxim Integrated │ 15
MAX17503 4.5V-60V, 2.5A, High-Efciency,
Synchronous Step-Down DC-DC Converter
With Internal Compensation
www.maximintegrated.com
Setting the Input Undervoltage-Lockout Level
Thedeviceoffersanadjustableinputundervoltage-lockout
level. Set the voltage at which the device turns on with
a resistive voltage-divider connected from V
IN
 to SGND.
Connect the center node of the divider to EN/UVLO.
ChooseR1tobe3.3MIandthencalculateR2asfollows:
×
=
INU
R1 1.215
R2
(V - 1.215)
where V
INU
is the voltage at which the device is required
to turn on. Ensure that V
INU
is higher than 0.8 x V
OUT
.
Loop Compensation
The device is internally loop compensated. However, if
theswitchingfrequencyislessthan500kHz, connect a
0402 capacitor C6 between the CF pin and the FB pin.
Use Table 2 to select the value of C6.
Iftheswitchingfrequencyislessthan 200kHz, connect
anadditionalR-Cnetworkinparalleltothetopresistorof
thefeedbackdivider(R3).SeeFigure5tocalculatethe
valuesofthecomponentsR7,C12,andC6.
Adjusting Output Voltage
Set the output voltage with a resistive voltage-divider
connected from the positive terminal of the output
capacitor (V
OUT
)toSGND(seeFigure3).Connectthe
center node of the divider to the FB pin. Use the following
procedure to choose the resistive voltage-divider values:
Calculate resistor R3 from the output to the FB pin as
follows:
3
C OUT
216 10
R3
fC
×
=
×
whereR3isinkΩ,crossoverfrequencyf
C
isinkHz,and
the output capacitor C
OUT
is in µF. Choose f
C
to be 1/9th
of the switching frequency, f
SW
, if the switching frequency
islessthanorequalto500kHz.Iftheswitchingfrequency
ismorethan500kHz,selectf
C
tobe55kHz.
CalculateresistorR4fromtheFBpintoSGNDasfollows:
×
=
OUT
R3 0.9
R4
(V - 0.9)
Power Dissipation
Ensure that the junction temperature of the device
does not exceed 125ºC under the operating conditions
specified for the power supply.
At a particular operating condition, the power losses that
lead to temperature rise of the part are estimated as
follows:
(
)
=××
η
2
LOSS OUT DCR
OUT
1
P (P ( - 1)) - I R
= ×
OUT OUT OUT
P VI
where P
OUT
is the total output power, η is the efficiency
of the converter, and R
DCR
is the DC resistances of the
inductor. (See the Typical Operating Characteristics for more
information on efficiency at typical operating conditions.)
For a multilayer board, the thermal performance metrics
for the package are given below:
JA
33 C Wθ=°
JC
2CWθ=°
Figure 2. Setting the Input Undervoltage Lockout
Figure 3. Setting the Output Voltage
Table 2. C6 Capacitor Value at Various
Switching Frequencies
SWITCHING FREQUENCY RANGE (kHz) C6 (pF)
200 to 300 2.2
300 to 400 1.2
400 to 500 0.75
R3
R4
SGND
FB
V
OUT
R1
R2
SGND
EN/UVLO
V
IN