LP3905 Datasheet, PDF (9/24 Pages) National Semiconductor (TI) – Power Management Unit For Low Power Handheld Applications

LP3905
www.ti.com
SNVS374D – JUNE 2006 – REVISED MAY 2013
Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown
protection.
The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the
input voltage is 3V or higher.
BUCK CONVERTER BLOCK DIAGRAM
EN
V_IN
SW
Undervoltage
Lockout
Ramp
Generator
Soft
Start
Thermal
Shutdown
Bandgap
2 MHz
Oscillator
Current Limit
Comparator
+
- Ref1
PFM Current
Comparator
+
- Ref2
Error
Amp
PWM Comparator
V_{REF +}
0.5V -
Vcomp +
1.0V -
pfm_low
pfm_hi
Control Logic
Adj Ver
Fixed Ver
FB
Frequency
Compensation
Driver
+
-
Zero Crossing
Comparator
GND
Figure 6. Simplified Functional Diagram
CIRCUIT OPERATION
The LP3905 Buck regulators operate as follows. During the first portion of each switching cycle, the control block
in the LP3905 turns on the internal PFET switch. This allows current to flow from the input through the inductor to
the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (V_IN–V_OUT)/L, by
storing energy in a magnetic field.
During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the
input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the
NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of - V_OUT/L.
The output filter stores charge when the inductor current is high, and releases it when inductor current is low,
smoothing the voltage across the load.
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