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參數(shù)資料
| 型號： | LTC3589HUJ-1#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 2600 kHz SWITCHING FREQ-MAX, PQCC40 |
| 封裝： | 6 X 6 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, QFN-40 |
| 文件頁數(shù)： | 15/46頁 |
| 文件大?。?/td> | 570K |
| 代理商： | LTC3589HUJ-1#PBF |
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LTC3589/LTC3589-1
22
3589fc
Operating Frequency
The switching frequency of each of the LTC3589/LTC3589-1
step-down switching regulators may be independently set
using I2C command register bits B1DTV2[5], B2DTV2[5]
and B3DTV2[5]. The power-on default frequency is
2.25MHz. Writing bit BxDTV2[5] HIGH will reduce the
switching frequency to 1.125MHz. Selection of the op-
erating frequency is determined by desired efciency,
component size and converter duty cycle.
Operation at lower frequency improves efciency by reduc-
ing internal gate charge and switching losses but requires
larger inductance and capacitance values for comparable
output ripple voltage. The lowest duty cycle of the step-
down switching regulator is determined by the converters
minimum on-time. Minimum on-time is the shortest time
durationthattheconverteriscapableofturningitstopPMOS
on and off again. The time consists of the gate charge time
OPERATION
plus internal delays associated with peak current sensing.
The minimum on-time of the LTC3589/LTC3589-1 is ap-
proximately 90ns. If the duty cycle falls below what can be
accommodated by the minimum on-time, the converter will
begin to skip cycles. The output voltage will continue to be
regulated but the ripple voltage and current will increase.
With the switching frequency set to 2.25MHz, the minimum
supported duty cycle is 20%. Switching at 1.125MHz the
converter can support a 10% duty cycle.
Phase Selection
To reduce the cycle by cycle peak current drawn by the
switchingregulators,theclockphaseofeachoftheLTC3589/
LTC3589-1 step-down switching regulators can be set
using I2C command register bits B1DTV2[6], B2DTV2[6]
and B3DTV2[6]. The internal full-rate clock has a nominal
duty cycle of 20% while the half-rate clocks have a 50%
duty cycle. Setting the command register bits high will
delay the start of each converter switching cycle by 20%
or 50% depending on the selected operating frequency.
Inductor Selection
The choice of step-down switching regulator inductor inu-
ences the efciency of the converter and the magnitude of
the output voltage ripple. Larger inductance values reduce
inductor current ripple and therefore lower output voltage
ripple. A larger value inductor improves efciency by low-
ering the peak current to be closer to the average output
current. Larger inductors, however, generally have higher
series resistance that counters the efciency advantage
of reduced peak current.
Inductor ripple current is a function of switching frequency,
inductance, VIN, and VOUT, as shown in this equation:
IL =
1
f L
VOUT 1–
VOUT
VIN
In an example application the LTC3589/LTC3589-1 step-
down switching regulator 3 has a maximum load of 1A,
VIN equals 3.8V, and VOUT is set for 1.2V. A good starting
design point for inductor ripple is 30% of output current
or 300mA. Using the equation for ripple current, a 1.2μH
inductor should be selected.
Table 7. Step-Down Switching Regulator 3 Command Register
Settings
COMMAND
REGISTER[BIT]
VALUE
SETTING
SCR1[5-4]
00*
01
10
Pulse-Skipping Mode
Burst Mode Operation
Forced Continuous Mode
OVEN[2]
0*
1
Disable
Enable
SCR2[2]
0*
1
Wait for Output Below 300mV Before Enable
Enable Immediately
VCCR[5]
0*
1
Select Register B3DTV1 (V1) Reference
Select Register B3DTV2 (V2) Reference
VCCR[4]
1
Initiate Dynamic Voltage Slew
VRRCR[5-4]
00
01
10
11*
Reference Slew Rate = 0.88mV/μs
Reference Slew Rate = 1.75mV/μs
Reference Slew Rate = 3.5mV/μs
Reference Slew Rate = 7mV/μs
B3DTV1[5]
0*
1
Force PGOOD Low When Slewing
Normal PGOOD Operation When Slewing
B3DTV1[4-0]
11001*
DAC Dynamic Target Voltage V1
B3DTV2[4-0]
11001*
DAC Dynamic Target Voltage V2
B3DTV2[5]
0*
1
2.25MHz Switching Frequency
1.125MHz Switching Frequency
B3DTV2[6]
0*
1
Switch on Clock Phase 1
Switch on Clock Phase 2
B3DTV2[7]
0*
1
Shutdown Regulator 3 Normally
Keep Regulator 3 Alive
* Denotes Default Power-On Value
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