LM3691TL-0.9 NSC [National Semiconductor], LM3691TL-0.9 Datasheet - Page 13

• Manufacturer Part Number: LM3691TL-0.9
• Description: High Accuracy, Miniature 1A, Step-Down DC-DC Converter for Portable Applications
• Manufacturer: NSC [National Semiconductor]

Operation Description
DEVICE INFORMATION
The LM3691, a high-efficiency, step-down DC-DC switching
buck converter, delivers a constant voltage from either a sin-
gle Li-Ion or three cell NiMH/NiCd battery to portable devices
such as cell phones and PDAs. Using a voltage mode archi-
tecture with synchronous rectification, the LM3691 has the
ability to deliver up to 1000 mA depending on the input voltage
and output voltage, ambient temperature, and the inductor
chosen.
There are three modes of operation depending on the current
required - PWM (Pulse Width Modulation), ECO, and shut-
down. The device operates in PWM mode at load currents of
approximately 50 mA (typ.) or higher. Lighter output current
loads cause the device to automatically switch into ECO
mode for reduced current consumption and a longer battery
life. Shutdown mode turns off the device, offering the lowest
current consumption (I
features include soft-start, under voltage protection, current
overload protection, and thermal shutdown protection. As
shown in Figure 1, only three external power components are
required for implementation.
CIRCUIT OPERATION
The LM3691 operates as follows. During the first portion of
each switching cycle, the control block in the LM3691 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
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 cur-
rent from ground through the NFET to the output filter capac-
itor and load, which ramps the inductor current down with a
slope of –V
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 rec-
tifier 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 operation, the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent load and line regulation.
The DC gain of the power stage is proportional to the input
voltage. To eliminate this dependence, feed forward inversely
proportional to the input voltage is introduced. While in PWM
mode, the output voltage is regulated by switching at a con-
stant 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 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.
IN
–V
OUT
)/L, by storing energy in a magnetic field. During the
OUT
/L.
SHUTDOWN
= 0.03 µA typ.). Additional
13
Internal Synchronous Rectification
While in PWM mode, the LM3691 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.
Current Limiting
A current limit feature allows the LM3691 to protect itself and
external components during overload conditions. PWM mode
implements current limit using an internal comparator that
trips at 1500 mA (typ). If the output is shorted to ground and
output voltage becomes lower than 0.3V (typ.), the device
enters a timed current limit mode where the switching fre-
quency will be one fourth, and NFET synchronous rectifier is
disabled, thereby preventing excess current and thermal run-
away.
ECO OPERATION
Setting mode pin low places the LM3691 in Auto mode. By
doing so the part switches from ECO (ECOnomy) state to
FPWM (Forced Pulse Width Modulation) state based on out-
put load current. At light loads (less than 50 mA), the converter
enters ECO mode. In this mode the part operates with low Iq.
During ECO operation, the converter positions the output
voltage slightly higher (+30 mV typ.) than the nominal output
voltage in FPWM operation. Because the reference is set
higher, the output voltage increases to reach the target volt-
age when the part goes from sleep state to switching state.
Once this voltage is reached the converter enters sleep mode,
thereby reducing switching losses and improving light load
efficiency. The output voltage ripple is slightly higher in ECO
mode (30 mV peak–peak ripple typ.).
FIGURE 4. Typical PWM Operation
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