LM3475 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – Hysteretic PFET Buck Controller

Design Information
SETTING OUTPUT VOLTAGE
The output voltage is programmed using a resistor divider
between V_OUT and GND as shown in Figure 2. The feedback
resistors can be calculated as follows:
Where Vfb is 0.8V typically.
The feedback resistor ratio, α = (R1+R2) / R2, will also be
used below to calculate output ripple and operating fre-
quency.
Where delay is the sum of the LM3475 propagation delay
time and the PFET delay time. The propagation delay is
90ns typically.
Minimum output ripple voltage can be determined using the
following equation:
V_{OUT_PP} = V_HYST ( R1 + R2 ) / R2
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 2. C_ff is connected in parallel with the high
side feedback resistor, R1. The output ripple causes a cur-
rent to be sourced or sunk through this capacitor. This cur-
rent 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 fre-
quency. 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:
FIGURE 2. Hysteretic Window
20070115
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 induc-
tance, 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 com-
ponents 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 pro-
cess 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 fre-
quency (F) can be approximately calculated using the for-
mula:
Where D is the duty cycle, defined as V_OUT/V_IN, and ∆I is the
allowable inductor ripple current.
Maximum allowable inductor ripple current should be calcu-
lated as a function of output current (I_OUT) as shown below:
∆I_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 + ∆I/2) x 1.1
The inductance value and the resulting ripple is one of the
key parameters controlling operating frequency.
OUTPUT CAPACITOR SELECTION
Once the desired operating frequency and inductance value
are selected, ESR must be selected based on the equation
in the Setting Operating Frequency and Output Ripple. This
process may involve a few iterations to select standard ESR
and inductance values.
In general, the ESR of the output capacitor and the inductor
ripple current create the output ripple of the regulator. How-
ever, the comparator hysteresis sets the first order value of
this ripple. Therefore, as ESR and ripple current vary, oper-
ating frequency must also vary to keep the output ripple
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