LM3661TL-1.25 NSC [National Semiconductor], LM3661TL-1.25 Datasheet - Page 13

• Manufacturer Part Number: LM3661TL-1.25
• Description: 450mA Subminiature, Micropower Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
• Manufacturer: NSC [National Semiconductor]

Frequency Synchronization
over/undershoot. Note that sharp edged signals from a pulse
or function generator can develop under/overshoot as high
as 10V at the end of an improperly terminated cable.
Over-voltage Protection
The LM3661 has an over-voltage comparator that prevents
the output voltage from rising too high when the device is left
in PWM mode under low-load conditions. Otherwise, the
output voltage could rise out of regulation from the minimum
energy transferred per cycle due to about 250 ns minimum
on-time of the PFET switch while in PWM mode. When the
output voltage rises by 70 mV over its regulation threshold,
the OVP comparator inhibits PWM operation to skip pulses
until the output voltage returns to the regulation threshold. In
over voltage protection, output voltage and ripple increase
slightly.
Shutdown Mode
Setting the EN input low, to SGND, places the LM3661 in a
0.5 µA (typ) shutdown mode. During shutdown, the PFET
switch, NFET synchronous rectifier, reference, control and
bias of the LM3661 are turned off. Setting EN high to V
enables normal operation. While turning on, soft start is
activated. EN is a Schmidt trigger digital input with thresh-
olds that are independent of the input voltage at V
must be set low to turn off the LM3661 during under voltage
conditions when the supply is less than the 2.7V minimum
operating voltage. The LM3661 is designed for mobile
phones and similar applications where power sequencing is
determined by the system controller and internal UVLO (Un-
der Voltage Lock Out) circuitry is unnecessary. The LM3661
has no UVLO circuitry. Although the LM3661 exhibits good
behavior while enabled at low input voltages, this is not
guaranteed.
Start-up
The LM3661 is designed to be started in LDO mode. Under
these conditions, the output voltage will increase at a rate
determined by the LDO current limit and the output capacitor
and load. This ramp time is typically about 600 µs. The
LM3661 may be started in PWM mode as well. Under these
conditions, the reference voltage for the error amplifier is
ramped up time is about 300µs and the output voltage will
follow. In this way, the input inrush current and output voltage
over shoot can be minimized.
Thermal Shutdown Protection
The LM3661 has thermal shutdown protection in PWM mode
to protect from short-term misuse and overload conditions.
When the junction temperature exceeds 150˚C, the device
shuts down and re-starts in soft start after the temperature
drops below 130˚C. Prolonged operation in thermal overload
conditions may damage the device and is considered bad
practice.
Current Limiting Protection
A current limit feature allows the LM3661 to protect itself and
external components during overload conditions. Current
limiting is implemented using an independent internal com-
(Continued)
DD
. EN
DD
13
parator. In PWM mode, cycle-by-cycle current limiting is
normally used. If an excessive load pulls the output voltage
down to approximately 0.45V, then the device switches to a
timed current limit mode. In timed current limit mode the
internal PFET switch is turned off after the current compara-
tor trips and the beginning of the next cycle is inhibited for
2.5 µs to force the instantaneous inductor current to ramp
down to a safe value. Timed current limit prevents the loss of
current control seen in some products when the output volt-
age is pulled low in serious overload conditions.
Application Information
PIN SELECTABLE OUTPUT
The LM3661 features pin-selectable output voltage to elimi-
nate the need for external feedback resistors. Select an
output voltage of 1.05V or 1.25V/1.35V/1.4V by setting the
V
V
driven off digitally by a logic gates that provide over 1.2V for
high state and less than 0.4V for a low state to ensure valid
logic levels. V
the input low, this pin must be set to a known state.
Isel Pin
Connecting the I
current limit comparator to low value and low (
to high value. Note that I
and this pin must connect to a known state of normal opera-
tion.
Table 1 shows selected I
Mode Transition
The LM3661 is designed to operate in two modes, LDO(Low
Dropout Regulator) mode for light load (15mA Max.) and
PWM Mode (Pulse Width Modulation) . As described in the
Device Operation Section, setting the SYNC/MODE pin low
yields LDO mode or high yields PWM mode. When mode
transitions from LDO to PWM and vice versa, harsh transient
conditions such as ramping the output load should be
avoided. To maintain a smooth transition, it is recommended
to keep the load to a minimum of 3mA or less for about 40us
before ramping into heavy load to avoid a large dip at the
output. Similarly, the same care must be applied when
changing output voltage from 1.05V to 1.25V/1.35V/1.40V
and vice versa (setting Vsel pin high or low) during full load.
Figure 3 below shows the mode transition from LDO to PWM
and PWM to LDO, and the load transient transition from light
load to heavy load is delayed by 40µs to allow the PWM loop
to respond properly.
SEL
DD
or low by connecting to GND. Alternatively, V
V
1.05V/1.25V
1.05V/1.25V
1.05V/1.35V
1.05V/1.35V
1.05V/1.40V
1.05V/1.40V
pin low or high. V
OUT
Table 1. I
(Applies to both V
option
SEL
SEL
SEL
input has no internal pull down that pulls
pin high (
condition and I
SEL
OUT
SEL
may be set high by connecting to
I
H
L
H
L
H
L
SEL
SEL
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capability information.
pin has no internal pull down
1.2V or Vin ) sets the internal
= H and V
OUT
I
300mA
TBDmA
350mA
450mA
350mA
450mA
OUT
capability
SEL
capability
<
= L)
0.4 or GND)
www.national.com
SEL
may be