LM2733YEVAL National Semiconductor, LM2733YEVAL Datasheet - Page 2

• Manufacturer Part Number: LM2733YEVAL
• Description: BOARD EVALUATION LM2733Y
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2733YEVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Current - Output: 1A
• Voltage - Input: 2.7 ~ 14V
• Regulator Topology: Boost
• Frequency - Switching: 600kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2733
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Voltage - Output: -
• Power - Output: -

www.national.com
Adjusting the Output Voltage
The output voltage is set using R2 and R3 as given by the
formula:
Solved for R2:
The evaluation board as shipped has a 13.3k resistor in-
stalled at R3. The appropriate value for R2 for any output
may be calculated from the above formula.
Feedforward Compensation
The feedforward capacitor C3 should be selected to set the
compensation zero at approximately 8 kHz. The value of C3
is calculated using:
The value of C3 is calculated after R2 is selected for the
output voltage needed for the specific application.
Guidelines for Component
Selection
Since it is assumed that some of the eval boards will be
modified to be used in different voltage and current configu-
rations, some guidelines are given to help select compo-
nents which are likely to be changed.
INDUCTOR L1: The amount of inductance required de-
pends on switching frequency, duty cycle and amount of
allowable ripple current. 10 µH is a good choice for most
applications. At low boost ratios such as 3.3V to 5V, the
LM2731 loop stability requires that the inductance not ex-
ceed 6.8 µH. Smaller inductors may be used in applications
with less output current. Higher ripple current resulting from
a smaller inductor means the maximum average current
(and power) will be less. Duty cycle also affects ripple cur-
rent, since the time the switch is ON determines the length of
time that the current has to ramp up. Any design must be
R2 = (V
V
C3 = 1 / (2 x π x 8k x R2)
OUT
= 1.23 (R2/R3) + 1.23
OUT
Good Stability
- 1.23) / 1.23 x R3
20069103
2
verified for maximum load current over the full temperature
range of the application to make sure the inductance is
sufficient.
Smaller inductors can be used (and make more sense eco-
nomically) if the load current is fairly light. The part may
operate in discontinuous mode (where inductor current
drops to zero during each switching cycle) using less induc-
tance, but this is harmless and actually increases stability
(phase margin) compared to continuous operation.
DIODE D1: Because of the fast switching speeds, a schottky
diode must be used for D1. The voltage rating (minimum)
should be at least 5V higher than the output voltage for safe
design margin. The average current rating of the diode
should be at least 50% more than the maximum output load
current of the application.
OUTPUT CAPACITOR C2: The output capacitor(s) used on
the LM273X must be good quality ceramics of the X7R or
X5R type. Z5U or Z5F types will not give sufficient capaci-
tance because of the applied voltage reducing effective ca-
pacitance.
The output capacitor is also critical for stability. As a basic
guideline, it is recommended for the LM2733: 4.7 µF mini-
mum, at output voltages of 10V or above. At lower output
voltages, use 10 - 22 µF. In general, the higher the load
current, the more output capacitance is required for stability.
For the LM2731: use at least 10 µF in 5V to 12V applications,
and use 22 µF at lower boost ratios (such as 3.3 to 5V).
Stability of the specific application should be verified over the
full operating temperature range by load step testing, where
the load current is increased from no load to full load
abruptly. This can be done simply by tapping the lead from
the load box onto the output terminal. The amount of ringing
seen on the output voltage waveform will define the stability
of the design:
Output Voltage Waveforms For
Stability Testing
Marginal Stability
20069104