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參數(shù)資料
| 型號(hào)： | LTC3810EG |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO28 |
| 封裝： | 5.30 MM, PLASTIC, SSOP-28 |
| 文件頁數(shù)： | 9/36頁 |
| 文件大?。?/td> | 495K |
| 代理商： | LTC3810EG |
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LTC3810
17
3810fb
the period. Since there is no transition loss term in the
synchronous MOSFET, optimal efciency is obtained by
minimizing RDS(ON)— by using larger MOSFETs or paral-
leling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower
RDS(ON) and meet the current and thermal requirements
if desired. The LTC3810 contains large low impedance
drivers capable of driving large gate capacitances without
signicantly slowing transition times. In fact, when driv-
ing MOSFETs with very low gate charge, it is sometimes
helpful to slow down the drivers by adding small gate
resistors (10
Ω or less) to reduce noise and EMI caused
by the fast transitions.
Operating Frequency
The choice of operating frequency is a tradeoff between
efciency and component size. Low frequency operation
improves efciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3810 applications is de-
termined implicitly by the one-shot timer that controls
the on-time, tON, of the top MOSFET switch. The on-time
is set by the current out of the ION pin and the voltage at
the VON pin according to:
tON =
VVON
IION
(76pF)
Tying a resistor RON from VIN to the ION pin yields an
on-time inversely proportional to VIN. For a step-down
converter, this results in approximately constant frequency
operation as the input supply varies:
f
=
VOUT
VVON RON(76pF)
[HZ]
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. The VON pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high VOUT applications, tie VON to INTVCC. Figures 7a
and 7b show how RON relates to switching frequency for
several common output voltages.
Changes in the load current magnitude will cause frequency
shift. Parasitic resistance in the MOSFET switches and
inductor reduce the effective voltage across the induc-
tance, resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be main-
tained. This is accomplished with a resistive divider from
the ITH pin to the VON pin and VOUT. The values required
will depend on the parasitic resistances in the specic
Figure 7a. Switching Frequency vs RON (VON = 0V)
Figure 7b. Switching Frequency vs RON (VON = INTVCC)
APPLICATIONS INFORMATION
RON (kΩ)
10
100
SWITCHING
FREQUENCY
(kHz)
1000
100
1000
3810 F07a
VOUT = 1.5V
VOUT = 5V
VOUT = 2.5V
VOUT = 3.3V
RON (kΩ)
10
100
SWITCHING
FREQUENCY
(kHz)
1000
100
1000
3810 F07b
VOUT = 3.3V
VOUT = 12V
VOUT = 5V
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