Datasheet

C1 =
I x t
ON
'V
0.4A x 2.28 Ps
1V
=
= 0.91 PF
LM5006
SNVS646B –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
C3: The capacitor on the V
CC
output provides not only noise filtering and stability, but its primary purpose is to
prevent false triggering of the V
CC
UVLO at the buck switch on/off transitions. C3 should be no smaller than 1 µF.
C2 and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its ESR,
ripple voltage due to its capacitance, and the nature of the load.
A low ESR for C2 is generally desirable so as to minimize power losses and heating within the capacitor.
However, the regulator requires a minimum amount of ripple voltage at the feedback input for proper loop
operation. For the LM5006 the minimum ripple required at pin 7 is 25 mV p-p, requiring a minimum ripple at V
OUT
of 100 mV for this example. Since the minimum ripple current (at minimum Vin) is 74 mA p-p, the minimum ESR
required at V
OUT
is 100 mV/74 mA = 1.35Ω. Since quality capacitors for SMPS applications have an ESR
considerably less than this, R3 is inserted as shown in the Block Diagram. R3’s value, along with C2’s ESR,
must result in at least 25 mV p-p ripple at pin 7. See the LOW OUTPUT RIPPLE CONFIGURATIONS section for
techniques to reduce the output ripple voltage.
D1/Q1: Either a Schottky diode or an N-Channel MOSFET may be used for the free-wheel switch. Use of a
MOSFET generally results in higher circuit efficiency. Other factors to consider are component dimensions (PC
board space) and cost. If a diode is to be used a power 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 important parameters are reverse recovery time and forward
voltage. The reverse recovery time determines how long the reverse current surge lasts with each turn-on of the
internal buck switch. The forward voltage drop affects efficiency. The diode’s reverse voltage rating must be at
least as great as the maximum input voltage, plus ripple and transients, and its current rating must be at least as
great as the maximum current limit specification. The diode’s average power dissipation is calculated from:
P
D1
= V
F
x I
OUT
x (1–D) (10)
Where V
F
is the diode’s forward voltage drop, and D is the on-time duty cycle. If a MOSFET is to be used for
synchronous rectification, an N-Channel device is required. The MOSFET’s voltage rating must be at least as
great as the maximum input voltage, plus ripple and transients, and its current rating must be at least as great as
the maximum current limit specification. The average power dissipation is calculated from:
P
Q1
= I
OUT
2 x R
DS(on)
x (1–D)
where
• R
DS(on)
is the device's on-resistance
• D is the on-time duty cycle (11)
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 the VIN pin suddenly increases 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 (400 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 1V (for this
exercise), C1 calculates to:
(12)
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 is 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.
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 LM5006.
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