LM3485_15 Datasheet, PDF (13/24 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3485
www.ti.com
SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013
Δi ≤ I_out * 0.386827 * I_out^{−0.366726}
for I_out > 2.0Amps
Δi ≤ I_out * 0.3
The inductance can be calculated based upon the desired operating frequency where:
V_IN - V_DS - V_{OUT D}
L=
x
'i
f
(7)
And
V_OUT + V_D
D = _{VIN - VDS + VD}
(8)
where D is the duty cycle, V_D is the diode forward voltage, and V_DS is the voltage drop across the PFET.
The inductor should be rated to the following:
Ipk = (Iout+Δi/2)*1.1
(9)
I_RMS =
iout² + ^'i2
3
(10)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The
second is the ESR.
OUTPUT CAPACITOR SELECTION (C_OUT)
The ESR of the output capacitor times the inductor ripple current is equal to the output ripple of the regulator.
However, the V_HYST sets the first order value of this ripple. As ESR is increased with a given inductance, then
operating frequency increases as well. If ESR is reduced then the operating frequency reduces.
The use of ceramic capacitors has become a common desire of many power supply designers. However,
ceramic capacitors have a very low ESR resulting in a 90° phase shift of the output voltage ripple. This results in
low operating frequency and increased output ripple. To fix this problem a low value resistor should be added in
series with the ceramic output capacitor. Although counter intuitive, this combination of a ceramic capacitor and
external series resistance provide highly accurate control over the output voltage ripple. The other types
capacitor, such as Sanyo POS CAP and OS-CON, Panasonic SP CAP, Nichicon "NA" series, are also
recommended and may be used without additional series resistance.
For all practical purposes, any type of output capacitor may be used with proper circuit verification.
INPUT CAPACITOR SELECTION (C_IN)
A bypass capacitor is required between the input source and ground. It must be located near the source pin of
the external PFET. The input capacitor prevents large voltage transients at the input and provides the
instantaneous current when the PFET turns on.
The important parameters for the input capacitor are the voltage rating and the RMS current rating. Follow the
manufacturer's recommended voltage derating. For high input voltage application, low ESR electrolytic capacitor,
the Nichicon "UD" series or the Panasonic "FK" series, is available. The RMS current in the input capacitor can
be calculated.
I_{RMS_CIN} = I_OUT* ^{(VOUT* (VIN - VOUT))1/2}
V_IN
(11)
The input capacitor power dissipation can be calculated as follows.
P_D(CIN) = I_{RMS_CIN2} * ESR_CIN
(12)
The input capacitor must be able to handle the RMS current and the P_D. Several input capacitors may be
connected in parallel to handle large RMS currents. In some cases it may be much cheaper to use multiple
electrolytic capacitors than a single low ESR, high performance capacitor such as OS-CON or Tantalum. The
capacitance value should be selected such that the ripple voltage created by the charge and discharge of the
capacitance is less than 10% of the total ripple across the capacitor.
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