LM26003MHX/NOPB National Semiconductor, LM26003MHX/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM26003MHX/NOPB
• Description: IC REG BUCK SW 3A 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM26003MHX/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 35 V
• Current - Output: 3A
• Frequency - Switching: 150kHz ~ 500kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Power - Output: 3.1W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM26003MHX

Design Information
EXAMPLE CIRCUIT
Figure 7
components have been selected based on the design criteria
given in the following sections.
SETTING OUTPUT VOLTAGE
The output voltage is set by the ratio of a voltage divider at
the FB pin as shown in the typical application. The resistor
values can be determined by the following equation:
INDUCTOR
The output inductor should be selected based on inductor
ripple current. The amount of inductor ripple current com-
pared to load current, or ripple content, is defined as Iripple/
Iload. Ripple content should be less than 40%. Inductor ripple
current, Iripple, can be calculated as shown below:
Larger ripple content increases losses in the inductor and re-
duces the effective current limit.
Larger inductance values result in lower output ripple voltage
and higher efficiency, but a slightly degraded transient re-
sponse. Lower inductance values allow for smaller case size,
but the increased ripple lowers the effective current limit
threshold.
Remember that inductor value also affects the sleep mode
threshold as shown in
shows a complete typical application schematic. The
Figure
3.
FIGURE 7. Example Circuit
3A, 300 kHz
13
Where Vfb = 1.236V typically.
A maximum value of 150 kΩ is recommended for the sum of
R1 and R2.
As input voltage decreases towards the nominal output volt-
age, the LM26003 can skip up to seven off-pulses as de-
scribed in the Low Vin Operation section. In low output voltage
applications, if the on-time reaches Ton
skip on-pulses to maintain regulation. There is no limit to the
number of pulses that are skipped. In this mode of operation,
however, output ripple voltage may increase slightly.
When choosing the inductor, the saturation current rating
must be higher than the maximum peak inductor current and
the RMS current rating should be higher than the maximum
load current. Peak inductor current, Ipeak, is calculated as:
For example, at a maximum load of 3A and a ripple content
of 10%, peak inductor current is equal to 3.15A which is safely
at the minimum current limit of 3.15A. By increasing the in-
ductor size, ripple content and peak inductor current are
lowered, which increases the current limit margin.
The size of the output inductor can also be determined using
the desired output ripple voltage, Vrip. The equation to deter-
mine the minimum inductance value based on Vrip is as
follows:
MIN
, the device will
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