LM34919_14 Datasheet, PDF (14/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
www.ti.com
a) Reduced ripple configuration: In Figure 10, Cff is added across R1 to AC-couple the ripple at V_OUT directly
to the FB pin. This allows the ripple at V_OUT to be reduced to a minimum of 25 mVpp by reducing R3, since the
ripple at V_OUT is not attenuated by the feedback resistors. The minimum value for Cff is determined from:
t_{ON (max)}
Cff =
(R1//R2)
(17)
where t_ON(max) is the maximum on-time, which occurs at V_IN(min). The next larger standard value capacitor should
be used for Cff. R1 and R2 should each be towards the upper end of the 2 kΩ to 10 kΩ range.
LM34919
L1
SW
Cff
FB
V_OUT
R1
R3
R2
C2
Figure 10. Reduced Ripple Configuration
b) Minimum ripple configuration: The circuit of Figure 11 provides minimum ripple at V_OUT, determined
primarily by C2's characteristics and the inductor's ripple current since R3 is removed. RA and CA are chosen to
generate a sawtooth waveform at their junction, and that voltage is AC-coupled to the FB pin via CB. To
determine the values for RA, CA and CB, use the following procedure:
Calculate V_A = V_OUT - (V_SW x (1 - (V_OUT/V_IN(min))))
(18)
where V_SW is the absolute value of the voltage at the SW pin during the off-time (typically 1V). V_A is the DC
voltage at the RA/CA junction, and is used in the next equation.
(V_IN(min) - V_A) x t_ON
RA x CA =
'V
(19)
where t_ON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the
RA/CA junction, typically 100 mV. RA and CA are then chosen from standard value components to satisfy the
above product. Typically CA is 3000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ. CB is then chosen large
compared to CA, typically 0.1 µF. R1 and R2 should each be towards the upper end of the 2 kΩ to 10 kΩ range.
LM34919
L1
SW
RA
CA
FB
CB
R1
V_OUT
C2
R2
Figure 11. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in Figure 12 is the same as that in Figure 7, except the
output voltage is taken from the junction of R3 and C2. The ripple at V_OUT is determined by the inductor's ripple
current and C2's characteristics. However, R3 slightly degrades the load regulation. This circuit may be suitable if
the load current is fairly constant.
14
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