LM3475_14 Datasheet, PDF (10/21 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3475
SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005
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SETTING OPERATING FREQUENCY AND OUTPUT RIPPLE
Although hysteretic control is a simple control scheme, the operating frequency and other performance
characteristics depend on external conditions and components. If the inductance, output capacitance, ESR, V_IN,
or C_ff is changed, there will be a change in the operating frequency and possibly output ripple. Therefore, care
must be taken to select components which will provide the desired operating range. The best approach is to
determine what operating frequency is desirable in the application and then begin with the selection of the
inductor and output capacitor ESR. The design process usually involves a few iterations to select appropriate
standard values that will result in the desired frequency and ripple.
Without the feedforward capacitor (C_ff), the operating frequency (F) can be approximately calculated using the
formula:
V_OUT
(V_IN - V_OUT) x ESR
F=
x
V_IN (V_HYST x D x L) + (V_IN x delay x ESR)
where
• Delay is the sum of the LM3475 propagation delay time and the PFET delay time
• The propagation delay is 90ns typically
(4)
Minimum output ripple voltage can be determined using the following equation:
V_{OUT_PP} = V_HYST ( R1 + R2 ) / R2
(5)
USING A FEED-FORWARD CAPACITOR
The operating frequency and output ripple voltage can also be significantly influenced using a speed up
capacitor, C_ff, as shown in Figure 13. C_ff is connected in parallel with the high side feedback resistor, R1. The
output ripple causes a current to be sourced or sunk through this capacitor. This current is essentially a square
wave. Since the input to the feedback pin (FB) is a high impedance node, the bulk of the current flows through
R2. This superimposes a square wave ripple voltage on the FB node. The end result is a reduction in output
ripple and an increase in operating frequency. When adding C_ff, calculate the formula above with α= 1. The value
of C_ff depends on the desired operating frequency and the value of R2. A good starting point is 1nF ceramic at
100kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is
programmed below 1.6V, the effect of C_ff will decrease significantly.
INDUCTOR SELECTION
The most important parameters for the inductor are the inductance and the current rating. The LM3475 operates
over a wide frequency range and can use a wide range of inductance values. Minimum inductance can be
calculated using the following equation:
V_IN - V_SD - V_OUT
L=
x
'I
D
F
where
• D is the duty cycle, defined as V_OUT/V_IN
• ΔI is the allowable inductor ripple current
(6)
Maximum allowable inductor ripple current should be calculated as a function of output current (I_OUT) as shown
below:
ΔI_max = I_OUT x 0.3
The inductor must also be rated to handle the peak current (I_PK) and RMS current given by:
I_PK = (I_OUT + ΔI/2) x 1.1
(7)
I_RMS =
I_OUT2
+
'I²
3
(8)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency.
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