LM34923 Datasheet, PDF (16/26 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923
SNVS695A – MARCH 2011 – REVISED FEBRUARY 2013
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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 LM34923 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 75 mA p-p, the minimum
ESR required at V_OUT is 100 mV/75 mA = 1.33Ω. 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 LOW OUTPUT RIPPLE CONFIGURATIONS for techniques
to reduce the output ripple voltage.
D1: 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.
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:
C1 =
I x t_ON
'V
0.4A x 2.28 Ps
=
1V
= 0.91 PF
(11)
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 LM34923.
UV and UVO pins: The Under Voltage Detector function is used to monitor a system voltage, such as the input
voltage at VIN, by connecting the UV pin to two resistors (R_UV1, R_UV2) as shown in the Block Diagram. When the
voltage at the UV pin increases above its threshold the UVO pin switches low. The UVO pin is high when the
voltage at the UV input pin is below its threshold. Hysteresis is provided by the internal 5µA current source which
is enabled when the voltage at the UV pin is below its threshold. The resistor values are calculated using the
following procedure:
Choose the upper and lower thresholds (V_UVH and V_UVL) at V_IN.
R_UV2
=
V_UVH - V_UVL
5 PA
=
V_UV(HYS)
5 PA
(12)
R_UV1
=
R_UV2 x 2.5V
V_UVL - 2.5V
(13)
16
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