NSC LM3677TL-1.8EV Datasheet

LM3677 Evaluation Board
National Semiconductor Application Note 1595 Jingdong Chen March 2007
Introduction
The LM3677 evaluation board is a working demonstration of a step down DC-DC converter. This application note contains information about the evaluation board. For further informa­tion on buck converter topology, device electrical character­istics, and component selection please refer to the datasheet.
General Description
The LM3677 converts high input voltages to lower output volt­ages with high efficiency through an inductor based switching topology. Automatic intelligent switching between PWM low­noise and PFM low-current mode offers improved system control. LM3677 is available in both fixed output voltage op­tions and adjustable voltage options range from 1.2V to 3.3V
in a micro SMD-5 package. The fixed output voltage options available in a SMD-5 package are 1.2V, 1.3V, 1.5V, 1.6V,
1.8V, 2.5V, 2.8V, and 3.3V.
Operating Conditions
Recommended input voltage range: VIN = 2.7V to 4.5V for 1.1V V
OUT
< 1.5V
VIN = 2.7V to 5.5V for 1.5V V
OUT
< 1.8V
VIN = (V
OUT
+ V
DROPOUT
) to 5.5V for 1.8V V
OUT
3.3V
where V
DROPOUT
= I
LOAD
* (R
DSON, PFET
+ R
INDUCTOR
)
Recommended load current: 0 mA I
OUT
600 mA
Ambient temperature (TA) range: -30C to +85C
Junction temperature (TJ) range: -30C to +125C
Typical Application
30012025
FIGURE 1. Fixed Output Voltage--Typical Application Circuit
30012026
FIGURE 2. Adjustable Output Voltage—Typical Application Circuit
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LM3677 Evaluation Board AN-1595
Output Voltage Selection for LM3677TL-ADJ
The output voltage of the adjustable parts can be pro­grammed through the resistor network connected from V
OUT
to FB to GND. The resistor from FB to GND (R2) should be 200k to keep the current drawn throught this network well below 16µA quiescent current level (PFM mode) but large enough that it is not susceptible to noise. If R2 is 200k, and given the VFB is 0.5V, then the current through the resistor feedback network will be 2.5µA. The ouput voltage of the ad­justable parts ranges from 1.2V to 3.3V. The output voltage formula is:
V
OUT
: output voltage (V)
VFB: feedback voltage (0.5V typical) R1: feedback resistor from V
OUT
to FB(Ω )
R2: feedback resistor from FB to GND (Ω) For the fixed output voltage parts the feedback resistors are
internal and R1 is 0Ω. The bypass capacitors C1and C2 (labeled C3 and C4 on Eval-
uation Board) in parallel with the feedback resistors are cho­sen for increased stability. Below are the formulas for C1and C2.
Table1. LM3677-ADJ Configurations for Various V
out
(Circuit of Figure 2)
V
OUT
(V)
R1(kΩ ) R2(kΩ )
C1(pF) C2(pF) L (µH) CIN (µF) C
OUT
(µF)
1.2 280 200 8.2 none 1.0 4.7 10
1.3 320 200 8.2 none 1.0 4.7 10
1.5 357 178 6.8 none 1.0 4.7 10
1.6 442 200 5.6 none 1.0 4.7 10
1.8 464 178 5.6 none 1.0 4.7 10
2.5 402 100 6.0 none 1.0 4.7 10
2.8 464 100 5.6 24 1.0 4.7 10
3.3 562 100 5.6 24 1.0 4.7 10
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AN-1595
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