Datasheet

+
+
+
V
IN
BST
RON
RTN
SW
VCC
FB
VIN
VOUT
R2
R7
R3
C9
C1
C4
R1
R5
L1
UVLO
C7
LM5019
12V - 95V
1
2
3
4
5
6
8
7
+
C5
1 F
0.1 F
127 NŸ
14 NŸ
237 NŸ
GND
(TP1)
(TP2)
UVLO/SD
U1
R6
(TP3)
(TP5)
GND
C8
+
D2
EXP
1 NŸ
6.98 NŸ
4.7 F
1.5Ÿ
220 H
0.01 F
1 F
0.1 F
SW
R
5
V
IN (UVLO,rising)
= 1.225V x
R
7
+1
¹
·
©
§
V
IN(HYS)
=
I
HYS
R
5
T
ON
=
V
IN
10
-10
x R3
Theory of Operation www.ti.com
2 Theory of Operation
Refer to the evaluation board schematic in Figure 2. When the circuit is in regulation, the buck switch is
turned on each cycle for a time determined by R3 and VIN according to the equation:
(1)
The on-time of this evaluation board ranges from 5.56µs at VIN = 12V to 702ns at VIN = 95V. The on-time
varies inversely with input voltage. At the end of each on-time the buck switch is off for at least 144ns. In
normal operation, the off-time is much longer. During the off-time, the load current is supplied by the
output capacitor (C9). When the output voltage falls sufficiently that the voltage at FB is below 1.225V, the
regulation comparator initiates a new on-time period. For stable, fixed frequency operation, a minimum of
25mV of ripple is required at FB to switch the regulation comparator. Refer to the LM5019 100V, 100mA
Constant On-Time Synchronous Buck Regulator (SNVS788) data sheet for a more detailed block diagram,
and a complete description of the various functional blocks.
3 UVLO
The UVLO resistors (R5, R7) are selected using the following two equations:
(2)
and
(3)
On this evaluation board R5=127kΩ and R7=14.0kΩ, resulting in UVLO rising threshold at VIN=12V and a
hysteresis of 2.5V.
Figure 2. Complete Evaluation Board Schematic for LM5019 Based Buck Converter
4 Board Connection and Start-up
The input connections are made to the TP1 (VIN) and TP2 (GND) terminals. The load is connected to the
TP3 (VOUT) AND TP5 (GND) terminals. Ensure the wires are adequately sized for the intended load
current. Before start-up a voltmeter should be connected to the input terminals, and to the 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 12V, at which time the output voltage should be 10V. If the
output voltage is correct, then increase the input voltage as desired and proceed with evaluating the
circuit. DO NOT EXCEED 100V AT VIN. Caution: Do not leave EVM powered when unattended.
2
AN-2238 LM5019 Buck Evaluation Board SNVA647A–May 2012–Revised April 2013
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