LM2593HVEVAL National Semiconductor, LM2593HVEVAL Datasheet - Page 2

• Manufacturer Part Number: LM2593HVEVAL
• Description: BOARD EVALUATION LM2593
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r

Specifications of LM2593HVEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 2A
• Voltage - Input: 4.5 ~ 60V
• Regulator Topology: Buck
• Frequency - Switching: 150kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2593
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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Component Selection
OUTPUT CAPACITOR
We have chosen a capacitor type similar to the input capaci-
tor mainly for logistic reasons. It was initially sized simply to
handle the RMS current as calculated below, and with a
voltage rating just higher than the output voltage. Subse-
quently, a Bode plot for the feedback loop confirmed that the
phase margin was acceptable at around 40˚. This validated
the initial selection. The required RMS rating of the output
capacitor is:
The capacitor we chose is 47 µF/16V with an RMS current
rating of 0.24A at 100 kHz and an ESR of 0.36Ω.
CATCH DIODE
The Voltage rating must be higher than the input voltage. We
have picked a 60V Schottky diode here. The average current
in the catch diode is
I
Layout Suggestions
a) Traces shown in BOLD need to be short (not wide) as they pass high frequency current pulses. Wide copper planes with switching current/voltage can
radiate excessively.
b) Trace to Feedback Pin (Pin 6) should not pass directly under L1 (to avoid pickup).
AVG_D
= I
O
• (1−D)
(Continued)
FIGURE 1.
2
I
Usually the average current would be a starting point for the
diode selection. But 60V Schottky diodes have a higher
forward voltage drop than low voltage Schottkys, unless they
are ‘over-sized’ in terms of their current rating. So to force
good efficiency, we wanted to consider a diode with a
‘hot-drop’ (the forward drop with the diode hot) of no greater
than 0.5V (at an instantaneous forward current of about 2A).
This meant using a 3A/60V Schottky diode from International
Rectifier.
Schematic
The board schematic is presented in Figure 1. The key
layout suggestions are also indicated on the schematic.
Shutdown capacity is available and the pinout marked ‘SD’
on the board can be taken low to cause the output of the
converter to fall to 0V. The ‘Flag’ pin output is also available
and it goes high (pulled up by R3 to the 12V rail) to indicate
that the output is well-regulated. When the output is ‘not OK’,
this pin is pulled down internally by the IC and in this condi-
tion it sinks 12V/21K=0.6 mA. The maximum voltage on the
Flag pin should not exceed 45V and the current into it should
not be higher than 3 mA. Therefore in our case it cannot be
connected directly to the input voltage rail. The resistors R1
and R2 from a simple voltage divider designed to give 1.23V
at the feedback pin when the output is at 12V.
AVG_D
= 2 • (1−0.27) = 1.47A
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