MAX17545 Datasheet, PDF (15/19 Pages) Maxim Integrated Products – Synchronous Step-Down DC-DC Converter

MAX17545
4.5V–42V, 1.7A, High-Efficiency,
Synchronous Step-Down DC-DC Converter
With Internal Compensation
V_IN
R1
EN/UVLO
R2
SGND
Figure 2. Setting the Input Undervoltage Lockout
Table 2. C6 Capacitor Value at Various
Switching Frequencies
SWITCHING FREQUENCY RANGE (kHz)
200 to 300
300 to 400
400 to 500
C6 (pF)
2.2
1.2
0.75
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 3.3MΩ and then calculate R2 as follows:
R2 = ^{R1× 1.215}
(V_INU - 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.
In the case the EN/UVLO pin is driven from an external
signal source, a series resistance of minimum 1kΩ is
recommended to be placed between the signal source
output and the EN/UVLO pin, to reduce voltage ringing
on the line.
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.
V_OUT
R3
FB
R4
SGND
Figure 3. Setting the Output Voltage
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:
R=3 39 × V_OUT
where R3 is in kΩ.
Calculate resistor R4 from the FB pin to SGND as follows:
R4 = ^{R3 × 0.9}
(V_OUT - 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:
( ) ^PLOSS
= (P_OUT ×
(
1
η
-
1))
-
I_OUT ² × R_DCR
P= _OUT V_OUT × I_OUT
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:
θ_JA =33°C W
θ_JC =2°C W
www.maximintegrated.com
Maxim Integrated │  15