Datasheet

R
1
V
IN (UVLO,rising)
= 1.225V x
R
7
+1
¹
·
©
§
V
IN(HYS)
=
I
HYS
R
1
T
ON
=
V
IN
10
-10
x R2
Theory of Operation www.ti.com
2 Theory of Operation
When the circuit is in regulation, the buck switch is turned on each cycle for a time determined by R3 and
V
IN
according to Equation 1:
(1)
The on-time of this evaluation board ranges from 1.95 µs at V
IN
= 12 V to 435 ns at V
IN
= 48 V. The on-
time varies inversely with input voltage. At the end of each on-time, the buck switch is off for at least 144
ns. In normal operation, the off-time is much longer. During the off-time, the load current is supplied by the
output capacitor (C6). When the output voltage falls sufficiently that the voltage at FB is below 1.225 V,
the regulation comparator initiates a new on-time period. For stable, fixed frequency operation, a minimum
of 25 mV of ripple is required at FB to switch the regulation comparator. For a more detailed block
diagram and a complete description of the various functional blocks, see the LM25017 48V, 650mA
Constant On-Time Synchronous Buck Regulator Data Sheet (SNVS783).
3 UVLO
The UVLO resistors (R1, R7) are selected using Equation 2:
(2)
and Equation 3:
(3)
On this evaluation board, R1 = 127 kΩ and R7 = 14.0 kΩ, resulting in UVLO rising threshold at V
IN
= 12 V
and a hysteresis of 2.5 V.
4 Board Connection and Start-up
The input connections are made to J2. The load is connected to J1. Ensure the wires are adequately
sized for the intended load current. Before start-up, a voltmeter should be connected to the input and
output terminals. The load current should be monitored with an ammeter or a current probe. It is
recommended that the input voltage be increased gradually to 12 V, at which time the output voltage
should be 10 V. If the output voltage is correct, increase the input voltage as desired and proceed with
evaluating the circuit. DO NOT EXCEED 48 V AT V
IN
(J2).
5 Bill of Materials (BOM)
Designator Value Description Package Part Number Manufacturer
Reference
CAP, CERM, 0.01uF, 16V, +/-10%, X7R, GRM188R71C103KA01 MuRata
C1 0.01uF
0603 D
CAP, CERM, 2.2uF, 50V, +10/%, X7R, 1206 1206 GRM31CR71H225KA88 MuRata
C2 2.2uF
L
CAP, CERM, 0.47uF, 50V, +10/%, X7R, 0805 GRM21BR71H474KA88 MuRata
C3 0.47uF
0805 L
CAP, CERM, 3300pF, 50V, +10/%, X7R, 0603 C0603C332K5RACTU Kemet
C4 3300pF
0603
CAP, CERM, 1uF, 25V, +10/%, X7R, 0603 0603 GRM188R71E105KA12 MuRata
C5 1uF
D
C6 10uF CAP, CERM, 10uF, 16V, +20/%, X7R, 1206 1206 C3216X7R1C106M TDK
CAP, CERM, 0.1uF, 100V, +10/%, X7R, 0603 GRM188R72A104KA35 MuRata
C7 0.1uF
0603 D
D1 0.65V Diode, Schottky, 40V, 0.35A, SOD-523 SOD-523 ZHCS350TA Diodes Inc.
INDUCTOR POWER 220UH 1A SMD 10mm x 7447714221 Wurth
L1 220uH
10mm Electronics Inc
R1 127k RES, 127k ohm, 1%, 0.1W, 0603 0603 CRCW0603127KFKEA Vishay-Dale
R2 237k RES, 237k ohm, 1%, 0.1W, 0603 0603 CRCW0603237KFKEA Vishay-Dale
2
LM25017 Evaluation Board SNVU228–April 2013
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