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參數(shù)資料
| 型號： | LM2608ATLX-1.8 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 400mA Sub-miniature, High Efficiency, Programmable DC-DC Converter with Linear Mode |
| 中文描述： | 0.69 A SWITCHING REGULATOR, 690 kHz SWITCHING FREQ-MAX, PBGA10 |
| 封裝： | MICRO, SMD-10 |
| 文件頁數(shù)： | 15/18頁 |
| 文件大?。?/td> | 728K |
| 代理商： | LM2608ATLX-1.8 |
Shutdown Mode
(Continued)
has no UVLO circuitry. Although the LM2608 exhibits good
behavior while enabled at low input voltages, this is not
guaranteed.
Internal Synchronous Rectification
The LM2608 uses an internal NFET as a synchronous rec-
tifier to improve efficiency by reducing rectifier forward volt-
age drop and associated power loss. In general, synchro-
nous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low com-
pared to the voltage drop across an ordinary rectifier diode.
Under moderate and heavy loads, the internal NFET syn-
chronous rectifier is turned on during the inductor current
down-slope in the second part of each cycle. The synchro-
nous rectifier is turned off prior to the next cycle, or when the
inductor current ramps near zero at light loads. The NFET is
designed to conduct through its intrinsic body diode during
transient intervals before it turns on, eliminating the need for
an external diode.
Current Limiting
A current limit feature allows the LM2608 to protect itself and
external components during overload conditions. Current
limiting is implemented using an independent internal com-
parator. In PWM mode, cycle-by-cycle current limiting is
normally used. If an excessive load pulls the output voltage
down to approximately 0.7V, then the device switches to a
timed current limit mode. In timed current limit mode the
internal P-FET switch is turned off after the current compara-
tor trips and the beginning of the next cycle is inhibited for
2.5μs 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.
Current Limiting and PWM Mode
Transient Response
Considerations
The LM2608 was designed for fast response to moderate
load steps. Harsh transient conditions during loads above
300mA can cause the inductor current to swing up to the
maximum current limit, resulting in PWM mode jitter or insta-
bility from activation of the current limit comparator. To avoid
this jitter or instability, do not power-up or start the LM2608
into a full load (loads near or above 400mA). Do not change
operating modes or output voltages when operating at a full
load. Avoid extremely sharp and wide-ranging load steps to
full load, such as from
<
30mA to
>
350mA.
Pin Selectable Output Voltage
The LM2608 features pin-selectable output voltage to elimi-
nate the need for external feedback resistors. The output
can be set to 1.3V, 1.5V or 1.8V by configuring the VSEL pin.
See
Setting the Output Voltage
in the
Application Information
section for further details.
Soft-Start
The LM2608 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 in the mS range. The
LM2608 may be started in PWM mode as well. Under these
conditions, the reference voltage for the error amp is ramped
up in about 100μs and the output voltage will follow. In this
way, the input inrush current and output voltage overshoot
can be minimized.
Thermal Shutdown Protection
The LM2608 has thermal shutdown protection in PWM mode
to protect from short-term misuse and overload conditions.
When the junction temperature exceeds 150C, the device
shuts down and re-starts in soft start after the temperature
drops below 130C. Prolonged operation in thermal overload
conditions may damage the device and is considered bad
practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM2608 features pin-selectable output voltage to elimi-
nate the need for external feedback resistors. Select an
output voltage of 1.3V, 1.5V or 1.8V by configuring the VSEL
pin, as directed in
Table 1
.
TABLE 1. VSEL Output Voltage Selection Settings
Output Voltage Options
LM2608 - 1.3 (300mA)
V
OUT
1.3V
1.5V
1.5V
1.8V
VSEL
GND
VDD
GND
VDD
LM2608 - 1.8 (400mA)
VSEL may be set high by connecting to VDD or low by
connecting to SGND. Optionally, VSEL may be driven by
digital gates that provide over 1.2V for a high state and less
than 0.4V for a low state to ensure valid logic levels. The
VSEL input has an internal 0.7 μA (typ) pull-down that pulls
the input low, when left unconnected. Leaving this pin open
is acceptable, but setting the pin high or low is recom-
mended.
INDUCTOR SELECTION
A 10μH inductor with a saturation current rating over the
current limit ( I
) of the LM2608 is recommended for most
applications. The inductor’s resistance should be less than
0.3
for good efficiency.
Table 2
lists suggested inductors
and suppliers.
TABLE 2. Suggested Inductors and Their Suppliers
Model
Vendor
Coilcraft
Coilcraft
Coiltronics
Coiltronics
Phone
FAX
DO1608C-103
DO1606T-103
UP1B-100
UP0.4CB-100
847-639-6400
847-639-1469
561-241-7876
561-241-9339
L
www.national.com
15
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