LM5005MHX Datasheet, PDF (14/28 Pages) Texas Instruments – LM5005 High Voltage 2.5 Amp Buck Regulator

LM5005
SNVS397D – SEPTEMBER 2005 – REVISED MARCH 2013
APPLICATION INFORMATION
www.ti.com
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. The Bill of
Materials for this design is listed in Table 1. The circuit shown in Figure 9 is configured for the following
specifications:
• V_OUT = 5V
• V_IN = 7V to 75V
• Fs = 300 KHz
• Minimum load current (for CCM) = 250 mA
• Maximum load current = 2.5A
R3 (R_T)
R_T sets the oscillator switching frequency. Generally, higher frequency applications are smaller but have higher
losses. Operation at 300KHz was selected for this example as a reasonable compromise for both small size and
high efficiency. The value of R_T for 300KHz switching frequency can be calculated as follows:
[(1 / 300 x 10³) ± 580 x 10^-9]
R_T =
135 x 10^-12
(3)
The nearest standard value of 21 kΩ was chosen for RT.
L1
The inductor value is determined based on the operating frequency, load current, ripple current, and the
minimum and maximum input voltage (V_IN(min), V_IN(max)).
IRIPPLE
I_PK+
I_O
I_PK-
0 mA
1/Fs
Figure 16. Inductor Current Waveform
To keep the circuit in continuous conduction mode (CCM), the maximum ripple current I_RIPPLE should be less
than twice the minimum load current, or 0.5 Ap-p. Using this value of ripple current, the value of inductor (L1) is
calculated using the following:
V_OUT x (V_IN(max) ± V_OUT)
L1 =
I_RIPPLE x F_S x V_IN(max)
(4)
5V x (75V ± 5V)
L1 = _{0.5A x 300 kHz x 75V} = 31 PH
(5)
This procedure provides a guide to select the value of L1. The nearest standard value (33 µH) will be used. L1
must be rated for the peak current (I_PK+) to prevent saturation. During normal loading conditions, the peak current
occurs at maximum load current plus maximum ripple. During an overload condition the peak current is limited to
3.5A nominal (4.25A maximum). The selected inductor (see Table 1) has a conservative 6.2 Amp saturation
current rating. For this manufacturer, the saturation rating is defined as the current necessary for the inductance
to reduce by 30%, at 20°C.
14
Submit Documentation Feedback
Product Folder Links: LM5005
Copyright © 2005–2013, Texas Instruments Incorporated