LM5088_15 Datasheet, PDF (20/39 Pages) Texas Instruments – Wide Input Range Non-Synchronous Buck Controller

LM5088, LM5088-Q1
SNVS600I – DECEMBER 2008 – REVISED AUGUST 2014
www.ti.com
9 Application and Implementation
9.1 Application Information
The LM5088 Wide Input Range buck Controller features all the functions necessary to implement an efficient
high voltage step-down converter using a minimum number of external components. The LM5088 is well suited
for a wide range of applications where efficient step-down of high, unregulated input voltage is required.
9.2 Typical Application
VIN (4.5V-75V)
RUV2
CIN
RUV1
CDITHER/RESTART
CVCC
RRAMP
CRAMP
CSS
EN
VIN _BOOT
HG
DITH/RES
SW
VCC
CS
LM5088
RAMP
CSG
SS
OUT
RT/SYNC
RRT
GND
FB
COMP
CBOOT
Q
L
D
COUT1
Rs
RCOMP ^CCOMP
VOUT
RFB2
COUT2 _RFB1
CHF
Figure 21. LM5088 Typical Application Schematic
9.3 Design Requirements
The procedure for calculating the external components is illustrated with the following design example. The
circuit shown in Figure 27 and Figure 28 is configured for the following specifications:
• Output Voltage = 5V
• Input Voltage = 5.5V to 36V
• Maximum Load Current = 7A
• Switching Frequency = 250 kHz
9.4 Detailed Design Procedure
9.4.1 Timing Resistor
The RT resistor sets the oscillator switching frequency. Higher frequencies result in smaller size components
such as the inductor and filter capacitors. However, operating at higher frequencies also results in higher
MOSFET and diode switching losses. Operation at 250 kHz was selected for this example as a reasonable
compromise between size and efficiency.
The value of RT resistor can be calculated as follows:
1 _{- 280 ns}
250 kHz
R_RT =
152 pF
= 24.5 k:
(8)
The nearest standard value of 24.9 kΩ was chosen for RT.
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