LM34919EVAL National Semiconductor, LM34919EVAL Datasheet - Page 14

• Manufacturer Part Number: LM34919EVAL
• Description: BOARD EVAL FOR LM34919 SW REG
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM34919EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 600mA
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 800kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34919
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where lower ripple at V
following options can be used to reduce or nearly eliminate
the ripple.
a) Reduced ripple configuration: In Figure 8, Cff is added
across R1 to AC-couple the ripple at V
pin. This allows the ripple at V
of 25 mVp-p by reducing R3, since the ripple at V
attenuated by the feedback resistors. The minimum value for
Cff is determined from:
where t
(min)
for Cff. R1 and R2 should each be towards the upper end of
the 2 kΩ to 10 kΩ range.
b) Minimum ripple configuration: The circuit of Figure 9
provides minimum ripple at V
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:
where V
during the off-time (typically 1V). V
RA/CA junction, and is used in the next equation.
where t
age), 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. Typ-
ically 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.
. The next larger standard value capacitor should be used
Calculate V
ON(max)
ON
FIGURE 8. Reduced Ripple Configuration
SW
is the maximum on-time (at minimum input volt-
is the absolute value of the voltage at the SW pin
is the maximum on-time, which occurs at V
A
= V
OUT
- (V
OUT
SW
OUT
to be reduced to a minimum
x (1 - (V
, determined primarily by
A
is the DC voltage at the
OUT
OUT
OUT
directly to the FB
/V
is required, the
IN(min)
30004425
OUT
)))
is not
IN
14
FIGURE 9. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in
Figure 10 is the same as that in Figure 5, except the output
voltage is taken from the junction of R3 and C2. The ripple at
V
characteristics. However, R3 slightly degrades the load reg-
ulation. This circuit may be suitable if the load current is fairly
constant.
Minimum Load Current
The LM34919 requires a minimum load current of 1 mA. If the
load current falls below that level, the bootstrap capacitor (C4)
may discharge during the long off-time, and the circuit will ei-
ther shutdown, or cycle on and off at a low frequency. If the
load current is expected to drop below 1 mA in the application,
R1 and R2 should be chosen low enough in value so they
provide the minimum required current at nominal V
PC BOARD LAYOUT
Refer to application note AN-1112 for PC board guidelines for
the Micro SMD package.
The LM34919 regulation, over-voltage, and current limit com-
parators are very fast, and respond to short duration noise
pulses. Layout considerations are therefore critical for opti-
mum performance. The layout must be as neat and compact
as possible, and all of the components must be as close as
possible to their associated pins. The two major current loops
have currents which switch very fast, and so the loops should
be as small as possible to minimize conducted and radiated
EMI. The first loop is that formed by C1, through the VIN to
SW pins, L1, C2, and back to C1.The second current loop is
formed by D1, L1, C2 and the SGND and ISEN pins.
The power dissipation within the LM34919 can be approxi-
mated by determining the total conversion loss (P
and then subtracting the power losses in the free-wheeling
diode and the inductor. The power loss in the diode is ap-
proximately:
where Iout is the load current, V
age drop, and D is the on-time duty cycle. The power loss in
the inductor is approximately:
OUT
is determined by the inductor’s ripple current and C2’s
FIGURE 10. Alternate Minimum Output Ripple
P
D1
Configuration
= Iout x V
F
F
is the diode’s forward volt-
x (1-D)
30004427
30004428
IN
OUT
- P
.
OUT
),