LM34917A Datasheet, PDF (12/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

where V_SW is the absolute value of the voltage at the SW pin
during the off-time (typically 1V). V_A, the DC voltage at the R3/
C8 junction, calculates to 4.63V, and is used in the next equa-
tion.
where t_ON is the maximum on-time (at minimum input volt-
age), and ΔV is the desired ripple amplitude at the R3/C8
junction, typically 100 mV. R3 and C8 are chosen from stan-
dard value components to satisfy the above product. For this
example, 3300 pF is chosen for C8, and 5.23 kΩ is chosen
for R3. C7 is chosen large compared to C8, typically 0.1 µF.
D1: A Schottky diode is recommended. Ultra-fast recovery
diodes are not recommended as the high speed transitions at
the SW pin may inadvertently affect the IC’s operation through
external or internal EMI. The diode must be rated for the max-
imum input voltage, the maximum load current, and the peak
current which occurs when the current limit and maximum
ripple current are reached simultaneously. The diode’s aver-
age power dissipation is calculated from:
P_D1 = V_F x I_OUT x (1-D)
where V_F is the diode’s forward voltage drop, and D is the on-
time duty cycle.
FINAL CIRCUIT
The final circuit is shown in Figure 4, and its performance is
shown in Figure 5 and Figure 6. Current limit measured ap-
proximately 1.34A at Vin = 8V, and 1.27A at Vin = 33V. The
output ripple amplitude measured 4 mVp-p at Vin = 8V, and
14 mVp-p at Vin = 33V.
FIGURE 4. Example Circuit
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FIGURE 5. Efficiency vs. Load Current and V_IN (Circuit of Figure 4)
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