Datasheet

C1 =
I x t
ON
'V
0.3A x 3.72 Ps
2.0V
=
= 0.56 PF
LM5008
www.ti.com SNVS280G –APRIL 2004–REVISED MARCH 2013
D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time
determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage
drop is significant in the event the output is short-circuited as it is only this diode’s voltage which forces the
inductor current to reduce during the forced off-time. For this reason, a higher voltage is better, although that
affects efficiency. A good choice is an ultrafast power diode, such as the MURA110T3 from ON Semiconductor.
Its reverse recovery time is 30ns, and its forward voltage drop is approximately 0.72V at 300 mA at 25°C. Other
types of diodes may have a lower forward voltage drop, but may have longer recovery times, or greater reverse
leakage. D1’s reverse voltage rating must be at least as great as the maximum Vin, and its current rating be
greater than the maximum current limit threshold (610 mA).
C1: This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage
ripple at Vin, on the assumption that the voltage source feeding Vin has an output impedance greater than zero.
At maximum load current, when the buck switch turns on, the current into pin 8 will suddenly increase to the
lower peak of the output current waveform, ramp up to the peak value, then drop to zero at turn-off. The average
input current during this on-time is the load current (300 mA). For a worst case calculation, C1 must supply this
average load current during the maximum on-time. To keep the input voltage ripple to less than 2V (for this
exercise), C1 calculates to:
(10)
Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the
capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and
voltage effects, a 1.0 µF, 100V, X7R capacitor will be used.
C4: The recommended value is 0.01µF for C4, as this is appropriate in the majority of applications. A high quality
ceramic capacitor, with low ESR is recommended as C4 supplies the surge current to charge the buck switch
gate at turn-on. A low ESR also ensures a quick recharge during each off-time. At minimum Vin, when the on-
time is at maximum, it is possible during start-up that C4 will not fully recharge during each 300 ns off-time. The
circuit will not be able to complete the start-up, and achieve output regulation. This can occur when the
frequency is intended to be low (e.g., R
ON
= 500K). In this case C4 should be increased so it can maintain
sufficient voltage across the buck switch driver during each on-time.
C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at V
IN
. A low
ESR, 0.1µF ceramic chip capacitor is recommended, located close to the LM5008.
FINAL CIRCUIT
The final circuit is shown in Figure 14. The circuit was tested, and the resulting performance is shown in Figure 7
through Figure 9.
MINIMUM LOAD CURRENT
A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level,
the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown, or cycle on
and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback
resistors should be chosen low enough in value so they provide the minimum required current at nominal Vout.
PC BOARD LAYOUT
The LM5008 regulation and over-voltage comparators are very fast, and as such will respond to short duration
noise pulses. Layout considerations are therefore critical for optimum performance. The components at pins 1, 2,
3, 5, and 6 should be as physically close as possible to the IC, thereby minimizing noise pickup in the PC tracks.
The current loop formed by D1, L1, and C2 should be as small as possible. The ground connection from C2 to
C1 should be as short and direct as possible.
If the internal dissipation of the LM5008 produces excessive junction temperatures during normal operation, good
use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of
the WSON-8 package can be soldered to a ground plane on the PC board, and that plane should extend out
from beneath the IC to help dissipate the heat. Additionally, the use of wide PC board traces, where possible,
can also help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along
with use of any available air flow (forced or natural convection) can help reduce the junction temperatures.
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