LMZ14203_13 Datasheet, PDF (13/25 Pages) Texas Instruments – LMZ14203 3A SIMPLE SWITCHER® Power Module with 42V Maximum Input Voltage

LMZ14203
www.ti.com
SNVS632N – DECEMBER 2009 – REVISED MARCH 2013
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 ΔV_IN be maintained then the
following equation may be used.
C_IN ≥ I_O * D * (1–D) / f_SW-CCM * ΔV_IN
(9)
If ΔV_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)
≥ 3.7μF
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)
(10)
This can be rearranged as
R_ON ≊ V_O / (1.3 * 10 ^-10 * f_SW(CCM))
(11)
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 LMZ14203 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
(12)
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)
(13)
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)
(14)
If R_ON calculated in Equation 11 is less than the minimum value determined in Equation 14 a lower frequency
should be selected. Alternatively, V_IN(MAX) can also be limited in order to keep the frequency unchanged.
Additionally note, the minimum off-time of 260 ns limits the maximum duty ratio. Larger R_ON (lower F_SW) should
be selected in any application requiring large duty ratio.
Discontinuous Conduction and Continuous Conduction Modes
At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the
critical conduction point, it will operate in continuous conduction mode (CCM). When operating in DCM the
switching cycle begins at zero amps inductor current; increases up to a peak value, and then recedes back to
zero before the end of the off-time. Note that during the period of time that inductor current is zero, all load
current is supplied by the output capacitor. The next on-time period starts when the voltage on the at the FB pin
falls below the internal reference. The switching frequency is lower in DCM and varies more with load current as
compared to CCM. Conversion efficiency in DCM is maintained since conduction and switching losses are
reduced with the smaller load and lower switching frequency. Operating frequency in DCM can be calculated as
follows:
f_SW(DCM)≊V_O*(V_IN-1)*6.8μH*1.18*10²⁰*I_O/(V_IN–V_O)*R_ON2
(15)
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