LM3687 Datasheet, PDF (18/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
Operation Description
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DEVICE INFORMATION
The LM3687 incorporates a high efficiency synchronous switching step-down DC-DC converter and a very low
dropout linear regulator.
The DC-DC converter delivers a constant voltage from a single Li- Ion battery and input voltage rails from 2.7V to
5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous
rectification, it has the ability to deliver up to 750mA load current depending on the input voltage, output voltage,
ambient temperature and the inductor chosen.
The linear regulator delivers a constant voltage biased from V_{IN_LIN} power input - typically the output voltage of
the DC-DC converter is used (post regulation) - with a maximum load current of 350mA.
Two enable pins allow the independent control of the two outputs. Shutdown mode turns off the device, offering
the lowest current consumption (I_SHUTDOWN = 0.1 µA typ).
Besides the shutdown feature, for the DC-DC converter there are two more modes of operation depending on the
current required:
- PWM (Pulse Width Modulation), and
- PFM (Pulse Frequency Modulation).
The device operates in PWM mode at load current of approximately 80 mA or higher. Lighter load currents cause
the device to automatically switch into PFM for reduced current consumption (I_{Q_VBATT} = 27 µA typ) and a longer
battery life.
Additional features include soft-start, startup mode of the linear regulator, under-voltage protection, current
overload protection, and over-temperature protection.
As shown in Figure 1: 'Typical Application Circuit: Linear Regulator as Post Regulator', only four external surface-
mount components are required for implementation -one inductor and three ceramic capacitors.
An internal reference generates 1.8V biasing an internal resistive divider to create a reference voltage range from
0.45V to 1.8V (in 50mV steps) for the linear regulator (depending on the output voltage setting defined in the fab)
and the 0.5V reference used for the DC-DC converter.
The Under-voltage lockout feature enables the device to startup once V_BATT has reached 2.65V typically and
turns the device off if V_BATT drops below 2.41V typically.
Note:
In the case that the DC-DC converter is switched off while the Linear Regulator is still enabled, an overshoot of
up to 150mV might appear at V_{OUT_LIN}, if all of the following conditions are present:
-high V_BATT
-down ramp on V_{IN_LIN} of greater than 100mV/16us taking the Linear Regulator into dropout
-light load on Linear Regulator
DC-DC CONVERTER OPERATION
During the first part of each switching cycle, the control block in the LM3687 turns on the internal PFET switch.
This allows current to flow from the input V_BATT 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 (V_BATT - V_{OUT_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 (- V_{OUT_DCDC} /
L).
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
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