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参数资料
| 型号： | LTC1929IG-PG |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A DUAL SWITCHING CONTROLLER, 310 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.209 INCH, PLASTIC, SSOP-28 |
| 文件页数： | 11/28页 |
| 文件大小： | 310K |
| 代理商： | LTC1929IG-PG |
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LTC1929/LTC1929-PG
significant amount of cycle skipping can occur with corre-
spondingly larger current and voltage ripple.
If an application can operate close to the minimum on-
time limit, an inductor must be chosen that has a low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement.
As a general
rule, keep the inductor ripple current of each phase equal
to or greater than 15% of IOUT(MAX) at VIN(MAX).
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1929 circuits: 1) LTC1929 VIN current (in-
cluding loading on the differential amplifier output),
2) INTVCC regulator current, 3) I2R losses and 4) Topside
MOSFET transition losses.
1) The VIN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the differential
amplifier output. VIN current typically results in a small
(<0.1%) loss.
2) INTVCC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from INTVCC to
ground. The resulting dQ/dt is a current out of INTVCC that
is typically much larger than the control circuit current. In
continuous mode, IGATECHG = (QT + QB), where QT and QB
are the gate charges of the topside and bottom side
MOSFETs.
APPLICATIO S I FOR ATIO
WU
U
If the external frequency (fPLLIN) is greater than the oscil-
lator frequency f0SC, current is sourced continuously,
pulling up the PLLFLTR pin. When the external frequency
is less than f0SC, current is sunk continuously, pulling
down the PLLFLTR pin. If the external and internal fre-
quencies are the same but exhibit a phase difference, the
current sources turn on for an amount of time correspond-
ing to the phase difference. Thus the voltage on the
PLLFLTR pin is adjusted until the phase and frequency of
the external and internal oscillators are identical. At this
stable operating point the phase comparator output is
open and the filter capacitor CLP holds the voltage. The
LTC1929 PLLIN pin must be driven from a low impedance
source such as a logic gate located close to the pin.
The loop filter components (CLP, RLP) smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components CLP and RLP determine how fast the loop
acquires lock. Typically RLP =10k and CLP is 0.01F to
0.1
F.
Minimum On-Time Considerations
Minimum on-time tON(MIN) is the smallest time duration
that the LTC1929 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty cycle
applications may approach this minimum on-time limit
and care should be taken to ensure that
t
V
Vf
ON MIN
OUT
IN
() < ()
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1929 will begin to skip
cycles resulting in nonconstant frequency operation. The
output voltage will continue to be regulated, but the ripple
current and ripple voltage will increase.
The minimum on-time for the LTC1929 is generally less
than 200ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases. This is of
particular concern in forced continuous applications with
low ripple current at light loads. If the duty cycle drops
below the minimum on-time limit in this situation, a
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