LP3905SD-A3EV National Semiconductor, LP3905SD-A3EV Datasheet - Page 2

• Manufacturer Part Number: LP3905SD-A3EV
• Description: BOARD EVALUATION LP3905SD-A3
• Manufacturer: National Semiconductor

Specifications of LP3905SD-A3EV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 4, Non-Isolated
• Voltage - Output: 1.5V, 1.85V, 2.8V, 3V
• Current - Output: 600mA, 600mA, 100mA, 100mA
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LP3905
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

www.national.com
Output Voltage Pre-set
For initial use the LP3905-A3 sample board has been pre-set
to 1.5V and 1.85V. The outputs of these two regulators may
be set via the selection of the external resistor network form-
ing the output feedback between the output voltage side of
the inductor and the feedback pins (FB1, FB2) of the device
and the feedback pins and ground. The user should thus alter
the values of R1, R2 and C7 to give the desired output voltage
on Buck1 and R3, R4, and C8 to change the output voltage
on Buck2.
Output Voltage Selection
The recommended resistance between FB and GND is
around 200kΩ. At this level the current through the resistor
Recommended Component Values for Specified Output Voltages
V
Adjustable Buck Components showing
OUT
1.85
1.0
1.2
1.4
1.5
1.6
2.5
2.8
3.3
(V)
Resistor Feedback Network
R
1F
200
280
357
365
442
536
402
464
562
(kΩ)
20209110
R
2F
200
200
200
182
200
200
100
100
100
2
(kΩ)
network is low but does not give rise to noise problems. The
feedback node voltage is 0.5V and hence a 200kΩ resistor
will result in a 2.5µA current.
The formula for output voltage selection is
For any output voltage greater than or equal to 1.0V, a ca-
pacitor should be added across R
capacitor is governed by the formula
The value of this capacitor may be chosen from the following
table giving the recommended component values for speci-
fied output voltages. C2 shown between the FB node and
GND is only required at higher output voltages.
V
V
R
R
OUT
FB
1F
2F
C1 (pF)
8.2
6.8
8.2
8.2
6.8
18
12
10
10
- output voltage (Volts)
- feedback voltage (0.5V)
- feedback resistor (V
FB)
- feedback resistor (FB to
GND)
1F
. The chosen value of the
20209112
20209111
C2 (pF)
none
none
none
none
none
none
none
33
33
OUT
to