LTC3637_15 Datasheet, PDF (18/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
Generally, the worst-case power dissipation is in dropout
at low input voltage. In dropout, the LTC3637 can provide
a DC current as high as the full 2.4A peak current to the
output. At low input voltage, this current flows through a
higher resistance MOSFET, which dissipates more power.
As an example, consider the LTC3637 in dropout at an
input voltage of 5V, a load current of 1A and an ambient
temperature of 85°C. From the Typical Performance graphs
of Switch On-Resistance, the R_DS(ON) of the top switch
at V_IN = 5V and 100°C is approximately 0.6Ω. Therefore,
the power dissipated by the part is:
P_D = (I_LOAD)² • R_DS(ON) = (1A)² • 0.6Ω = 0.6W
For the MSOP package the θ_JA is 45°C/W. Thus, the junc-
tion temperature of the regulator is:
T_J
=
85°C
+
0.6W
•
45°C
W
=
112°C
which is below the maximum junction temperature of
150°C.
Note that the while the LTC3637 is in dropout, it can provide
output current that is equal to the peak current of the part.
This can increase the chip power dissipation dramatically
and may cause the internal overtemperature protection
circuitry to trigger at 180°C and shut down the LTC3637.
Therefore, the minimum inductor requirement is satisfied
and the 4.7μH inductor value may be used.
Next, C_IN and C_OUT are selected. For this design, C_IN should
be sized for a current rating of at least:
I_RMS
=
1A
•
3.3V
24V
•
24V
3.3V
–
1
≅
350mA_RMS
The value of C_IN is selected to keep the input from droop-
ing less than 240mV (1%):
C_IN
>
4.7µH
2 • 24V
•
•
2.4A²
240mV
≅
2.2µF
C_OUT will be selected based on a value large enough to
satisfy the output voltage ripple requirement. For a 50mV
output ripple, the value of the output capacitor can be
calculated from:
C_OUT
>
4.7µH • 2.4A²
2 • 3.3V • 50mV
≅
100µF
C_OUT also needs an ESR that will satisfy the output voltage
ripple requirement. The required ESR can be calculated from:
ESR
<
50mV
2.4A
≅
20mΩ
Design Example
As a design example, consider using the LTC3637 in an ap-
plication with the following specifications: typical V_IN = 24V,
maximum applied V_IN = 80V, V_OUT = 3.3V, I_OUT = 1A,
f = 200kHz. Furthermore, assume for this example that
switching should start when V_IN is greater than 6V and
stop switching when V_IN is greater than 48V.
First, calculate the inductor value that gives the required
switching frequency:
L
=


3.3V 
200kHz • 2.4A ^
•


1–
3.3V
24V


≅
4.7µH
Next, verify that this value meets the L_MIN requirement.
For this input voltage and peak current, the minimum
inductor value is:
A 100µF ceramic capacitor has significantly less ESR
than 20mΩ.
Since an output voltage of 3.3V is one of the standard
output configurations, the LTC3637 can be configured
by connecting V_PRG1 to ground and V_PRG2 to the SS pin.
The undervoltage and overvoltage lockout requirements
on V_IN can be satisfied with a resistive divider from V_IN to
the RUN and OVLO pins (refer to Figure 9). Pick R_TOTAL
= 1M = R3 + R4 + R5 to minimize the loading on V_IN and
calculate R3, R4 and R5 as follows (standard values):
R5
=
1M
•
1.21V
48V
=
24.9k
R4 =
1M •
1.21V
6V
–
24.9k
=
174k
L_MIN
=
48V • 150ns
2.4A
•
1.2
≅
4µH
R3 = 1M− 24.9k –174k =806k
3637fa
18
For more information www.linear.com/LTC3637