參數(shù)資料
型號: LTC1148HVCS-5#TR
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO14
封裝: 0.150 INCH, PLASTIC, SOP-14
文件頁數(shù): 19/20頁
文件大?。?/td> 237K
代理商: LTC1148HVCS-5#TR
8
LTC1148
LTC1148-3.3/LTC1148-5
114835fd
APPLICATIO S I FOR ATIO
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UU
U
The basic LTC1148 series application circuit (fixed
output versions) is shown in Figure 1. External compo-
nent selection is driven by the load requirement, and
begins with the selection of RSENSE. Once RSENSE is
known, CT and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, CIN and COUT are
selected and the loop is compensated. The circuit
shown in Figure 1 can be configured for operation up to
an input voltage of 20V. If the application requires
higher input voltage, then the LTC1149 or LTC1159
should be used.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
The LTC1148 series current comparator has a threshold
range which extends from a minimum of 25mV/RSENSE to
a maximum of 150mV/RSENSE. The current comparator
threshold sets the peak of the inductor ripple current,
yielding a maximum output current IMAX equal to the peak
value less half the peak-to-peak ripple current. For proper
Burst Mode operation, IRIPPLE(P-P) must be less than or
equal to the minimum current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
IRIPPLE(P-P) = 25mV/RSENSE (See CT and L Selection for
Operating Frequency). Solving for RSENSE and allowing
a margin for variations in the LTC1148 series and
external component values yields:
RSENSE =
100mV
IMAX
A graph for selecting RSENSE versus maximum output
current is given in Figure 2.
The load current below which Burst Mode operation com-
mences (IBURST)andthepeakshort-circuitcurrent(ISC(PK))
both track IMAX. Once RSENSE has been chosen, IBURST and
ISC(PK) can be predicted from the following:
IBURST
15mV
RSENSE
ISC(PK) =
150mV
RSENSE
Figure 2. Selecting RSENSE
MAXIMUM OUTPUT CURRENT (A)
0
R
SENSE
(
)
0.15
0.20
4
LTC1148 F02
0.10
0.05
0
1
2
3
5
The LTC1148 series automatically extends tOFF during a
short circuit to allow sufficient time for the inductor
current to decay between switch cycles. The resulting
ripple current causes the average short-circuit current
ISC(AVG) to be reduced to approximately IMAX.
L and CT Selection for Operating Frequency
The LTC1148 series uses a constant off-time architecture
with tOFF determined by an external timing capacitor CT.
Each time the P-channel MOSFET switch turns on, the
voltage on CT is reset to approximately 3.3V. During the off
time, CT is discharged by a current which is proportional
to VOUT. The voltage on CT is analogous to the current in
inductor L, which likewise decays at a rate proportional to
VOUT. Thus the inductor value must track the timing
capacitor value.
The value of CT is calculated from the desired continuous
mode operating frequency, f:
CT =
1
2.6(104)f
Assumes VIN = 2VOUT, Figure 1 circuit.
A graph for selecting CT versus frequency including the
effects of input voltage is given in Figure 3.
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The complete expression for operating
frequency of the circuit in Figure 1 is given by:
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