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LM3686 Datasheet(PDF) 14 Page - Texas Instruments

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Part No. LM3686
Description  Step-Down DC-DC Converter with Integrated Post Linear Regulators System and Low-Noise Linear Regulator
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3686 Datasheet(HTML) 14 Page - Texas Instruments

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TIME (330 ns/DIV)
VSW
IL
VOUT
2 mV/DIV
AC Coupled
200 mA/DIV
2V/DIV
IOUT = 500 mA
VBATT = 3.6V
VOUT = 1.8V
LM3686
SNVS520D – AUGUST 2008 – REVISED MARCH 2011
www.ti.com
In the case that the DC-DC converter is switched off while the Linear Regulator is still enabled, the LILO can still
support up to 50 mA. The linear regulator LILO is turned on via a small NMOS device supplied by VIN_LDO . The
maximum current is 50 mA when this small NMOS is ON. If higher current > 50 mA is desired the following
condition must be done:
1) EN_DC = HIGH
When the condition is met, the LILO transitions to the large NMOS and can support up to 350 mA.
DC-DC CONVERTER OPERATION
During the first part of each switching cycle, the control block in the LM3686 turns on the internal PFET switch.
This allows current to flow from the input VBATT through the switch pin SW and the inductor to the output filter
capacitor and load. The inductor limits the current to a ramp with a slope of (VBATT - VOUT_DCDC) / L, by storing
energy in the magnetic field.
During the second part 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 (- VOUT_DCDC /
L).
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the
load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and
synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The
output voltage is equal to the average voltage at the SW pin.
PWM Operation
During PWM (Pulse Width Modulation) operation the converter operates as a voltage-mode controller with input
voltage feed forward. This allows the converter to achieve good load and line regulation. The DC gain of the
power stage is proportional to the input voltage. To eliminate this dependency, feed forward inversely
proportional to the input voltage is introduced.
While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating
the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned
on and the inductor current ramps up until the duty-cycle-comparator trips and the control logic turns off the
switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is
exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by
the clock turning off the NFET and turning on the PFET.
Figure 5. Typical PWM Operation
Internal Synchronous Rectification
While in PWM mode, the DC-DC converter uses an internal NFET as a synchronous rectifier to reduce rectifier
forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier
diode.
14
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