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參數(shù)資料
| 型號： | LTC1626 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Low Voltage, High Efficiency Step-Down DC/DC Converter(低壓，高效率，步降DC/DC變換器) |
| 中文描述： | 低，高效率步電壓型DC / DC轉(zhuǎn)換器（低壓，高效率，步降直流/直流變換器） |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 242K |
| 代理商： | LTC1626 |
5
LTC1626
OPERATIOU
The nominal off-time of the LTC1626 is set by an external
timing capacitor connected between the C
T
pin and ground.
The operating frequency is then determined by the off-
time and the difference between V
IN
and V
OUT
.
The output voltage is set by an external divider returned to
the V
FB
pin. A voltage comparator V and a gain block G
compare the divided output voltage with a reference
voltage of 1.25V.
To optimize efficiency, the LTC1626 automatically switches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is heavy, the LTC1626 is in continuous
operation. During the switch “ON” time, current compara-
tor C monitors the voltage between the SENSE
+
and
SENSE
–
pins connected across an external shunt in series
with the inductor. When the voltage across the shunt
reaches the comparator’s threshold value, its output sig-
nal changes state, resetting the flip-flop and turning the
internal P-channel MOSFET off. The timing capacitor
connected to the C
T
pin is now allowed to discharge at a
rate determined by the off-time controller.
When the voltage on the timing capacitor has discharged
past V
TH1
, comparator T trips, sets the flip-flop and causes
the switch to turn on. Also, the timing capacitor is
recharged. The inductor current will again ramp up until
the current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage
decreases slightly. This causes the output of the gain stage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
When the load is relatively light, the LTC1626 automati-
cally switches to Burst Mode operation. The current loop
is interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator V trips
when V
OUT
is above the desired output voltage, turning off
the switch and causing the timing capacitor to discharge.
This capacitor discharges past V
TH1
until its voltage drops
below V
TH2
. Comparator S then trips and a sleep signal is
generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1626 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 1.9mA to 165
μ
A. When the output
capacitor discharges by the amount of the hysteresis of
the comparator V, the P-channel switch turns on again and
the process repeats itself. During Burst Mode operation,
the peak inductor’s current is set at 25mV/R
SENSE
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V
OS
is incorporated
in the gain stage. This prevents the current from increas-
ing until the output voltage has dropped below a minimum
threshold.
In dropout, the P-channel MOSFET is turned on continu-
ously (100% duty cycle) providing low dropout operation
with V
OUT
V
IN
.
APPLICATIO
S I
N
FOR
ATIO
U
The basic LTC1626 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, C
T
and L can be chosen. Next, the
Schottky diode D1 is selected followed by C
IN
and C
OUT
.
W
U
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the peak inductor current. Depending upon
the load current condition, the threshold of the compara-
tor lies between 25mV/R
SENSE
and 150mV/R
SENSE
. The
maximum output current of the LTC1626 is:
I
OUT(MAX)
= 150mV/R
SENSE
– I
RIPPLE
/2 (A)
Where I
RIPPLE
is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1626 is in Burst Mode
operation. In this mode, the peak current is set at 25mV/
R
SENSE
. To fully benefit from Burst Mode operation, the
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