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LMZ14203EXT
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SNVS666F – JUNE 2010 – REVISED OCTOBER 2013
Solving:
C O ≥ 3A*0.8V*6.8 μ H*24V / (4*3.3V*( 24V - 3.3V)*33mV)
≥ 43 μ F
The LMZ14203EXT demonstration and evaluation boards are populated with a 100 uF 6.3V X5R output
capacitor. Locations for extra output capacitors are provided.
C IN SELECTION
The LMZ14203EXT module contains an internal 0.47 μF input ceramic capacitor. Additional input capacitance is
required external to the module to handle the input ripple current of the application. This input capacitance should
be located in very close proximity to the module. Input capacitor selection is generally directed to satisfy the input
ripple current requirements rather than by capacitance value. Worst case input ripple current rating is dictated by
the equation:
I(C IN(RMS) ) ? 1 /2 * I O * √ (D / 1-D)
where
?
D ? V O / V IN
(5)
(As a point of reference, the worst case ripple current will occur when the module is presented with full load
current and when V IN = 2 * V O ).
Recommended minimum input capacitance is 10uF X7R ceramic with a voltage rating at least 25% higher than
the maximum applied input voltage for the application. It is also recommended that attention be paid to the
voltage and temperature deratings of the capacitor selected. It should be noted that ripple current rating of
ceramic capacitors may be missing from the capacitor data sheet and you may have to contact the capacitor
manufacturer for this rating.
If the system design requires a certain minimum value of input ripple voltage Δ V IN be maintained then the
following equation may be used.
C IN ≥ I O * D * (1–D) / f SW-CCM * Δ V IN
(6)
If Δ V IN is 1% of V IN for a 24V input to 3.3V output application this equals 240 mV and f SW = 400 kHz.
C IN ≥ 3A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V)
≥ 3.7 μ F
Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input
capacitance and parasitic inductance of the incoming supply lines.
R ON RESISTOR SELECTION
Many designs will begin with a desired switching frequency in mind. For that purpose the following equation can
be used.
f SW(CCM) ? V O / (1.3 * 10 -10 * R ON )
This can be rearranged as
R ON ? V O / (1.3 * 10 -10 * f SW(CCM) )
(7)
(8)
The selection of RON and f SW(CCM) must be confined by limitations in the on-time and off-time for the COT
Control Circuit Overview section.
The on-time of the LMZ14203EXT timer is determined by the resistor R ON and the input voltage V IN . It is
calculated as follows:
t ON = (1.3 * 10 -10 * R ON ) / V IN
(9)
The inverse relationship of t ON and V IN gives a nearly constant switching frequency as V IN is varied. R ON should
be selected such that the on-time at maximum V IN is greater than 150 ns. The on-timer has a limiter to ensure a
minimum of 150 ns for t ON . This limits the maximum operating frequency, which is governed by the following
equation:
f SW(MAX) = V O / (V IN(MAX) * 150 nsec)
(10)
This equation can be used to select R ON if a certain operating frequency is desired so long as the minimum on-
time of 150 ns is observed. The limit for R ON can be calculated as follows:
R ON ≥ V IN(MAX) * 150 nsec / (1.3 * 10 -10 )
(11)
Copyright ? 2010–2013, Texas Instruments Incorporated
Product Folder Links: LMZ14203EXT
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