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參數(shù)資料
| 型號(hào)： | LTC3538EDCB#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.8 A SWITCHING REGULATOR, 1200 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 2 X 3 MM, PLASTIC, DFN-8 |
| 文件頁數(shù)： | 16/16頁 |
| 文件大?。?/td> | 206K |
| 代理商： | LTC3538EDCB#TR |
LTC3538
9
3538fb
OPERATION
Depending on the VC voltage, the LTC3538 will operate in
either buck, buck-boost or boost mode. The four power
switches are properly phased so the transfer between
operating modes is continuous, smooth and transparent to
the user. When VIN approaches VOUT the buck-boost region
is entered, where the conduction time of the four-switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
Buck-Boost or Four Switch (VIN ~ VOUT)
When the control voltage, VC, is above voltage V2, switch
pair AD remains on for duty cycle DMAX_BUCK,andtheswitch
pair AC begins to phase in. As switch pair AC phases in,
switch pair BD phases out accordingly. When VC reaches
the edge of the buck-boost range, at voltage V3, the AC
switch pair completely phase out the BD pair, and the boost
phase begins at duty cycle D4SW. The input voltage, VIN,
where the four switch region begins is given by:
VIN = VOUT(1 – D4SW) ≈ 0.85 VOUT
The point at which the four-switch region ends is given
by:
V
IN =
V
OUT
1 D4SW
V ≈ 1.18 VOUT
Boost Region (VIN < VOUT)
Switch A is always on and switch B is always off during
this mode. When the control voltage, VC, is above volt-
age V3, switch pair CD will alternately switch to provide
a boosted output voltage. This operation is typical to a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 88% typical and is reached
when VC is above V4.
Burst Mode OPERATION
Burst Mode operation reduces quiescent current consump-
tion of the LTC3538 at light loads and improves overall
conversion efciency, increasing battery life. During Burst
Mode operation the LTC3538 delivers energy to the out-
put until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 35μA. In this mode the output ripple has a variable
frequency component that depends upon load current,
and will typically be about 2% peak-to-peak. Burst Mode
operation ripple can be reduced slightly by using more
output capacitance. Another method of reducing Burst
Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the VOUT feedback
divider network (as in Type III compensation).
3538 F02
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
FOUR-SWITCH PWM
BUCK-BOOST
REGION
D ON, C OFF
PWM A, B SWITCHES
DMIN
BOOST
DMAX
BUCK
DMAX
BOOST
0%
88%
DUTY
CYCLE
V3 (~1.8V)
V2 (~1.7V)
V4 (~2.2V)
V1 (~1.2V)
CONTROL
VOLTAGE, VC
BUCK REGION
Figure 2. Switch Control vs Control Voltage, VC
Buck Region (VIN > VOUT)
Switch D is always on and switch C is always off during
this mode. When the control voltage, VC, is above volt-
age V1, output A begins to switch. During the off time of
switch A, synchronous switch B turns on for the remainder
of the period. Switches A and B will alternate similar to a
typical synchronous buck regulator. As the control volt-
age increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches DMAX_BUCK, given by:
DMAX_BUCK = 100 – D4SW %
where D4SW = duty cycle % of the four switch range.
D4SW = (150ns f) 100 %
where f = operating frequency, Hz.
Beyond this point the four switch, or buck-boost region
is reached.
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