LP3905SD-A3EV National Semiconductor, LP3905SD-A3EV Datasheet - Page 10

• 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: -

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Functional Description
the cycle repeats to restore the output voltage (average
voltage in pfm mode) to ∼1.15% above the nominal PWM
output voltage.
If the load current should increase during PFM mode caus-
ing the output voltage to fall below the ‘low2’ PFM threshold,
the part will automatically transition into fixed-frequency
SOFT START
The LP3905 Buck Converters have a soft-start circuit that
limits in-rush current during start-up. Additionally, in case
EN1 and EN2 are enabled at the same time, a typical 500µs
delay between Buck1 and Buck2 Power Up prevents any
further Inrush current from the battery.
During start-up the switch current limit is increased in steps.
Soft start is activated only if EN goes from logic low to logic
FIGURE 10. Operation in PFM Mode and Transfer to PWM Mode
(Continued)
10
PWM mode. When V
to PFM mode at ~ 35mA output current and from PFM to
PWM mode at ~ 85mA , when V
sition happens at ~ 50mA and PFM to PWM transition hap-
pens at ~ 100mA, when V
happens at ~ 65mA and PFM to PWM transition happens at
~ 115mA.
high after Vin reaches 3V. Soft start is implemented by
increasing switch current limit in steps of 70mA, 140mA,
280mA and 1000mA (typ. switch current limit). The start-up
time thereby depends on the output capacitor and load
current demanded at start-up. Typical start-up times with
22µF output capacitor and 300mA load current is 400µs and
with 1mA load current its 275µs.
IN
=2.8V the part transitions from PWM
IN
=4.5V, PWM to PFM transition
IN
=3.6V, PWM to PFM tran-
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