參數(shù)資料
型號(hào)： LTC3823IGN#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 4 A SWITCHING CONTROLLER, PDSO28
封裝： 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28
文件頁(yè)數(shù)： 8/26頁(yè)
文件大?。?/td> 302K
代理商： LTC3823IGN#TRPBF
LTC3823
16
3823fd
INTVCC Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3823. The INTVCC pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum
of 4.7μF low ESR tantalum capacitor or other low ESR
capacitor. Good bypassing is necessary to supply the high
transient currents required by the MOSFET gate drivers.
Applications using large MOSFETs with a high input voltage
and high frequency of operation may cause the LTC3823
to exceed its maximum junction temperature rating or
RMS current rating. Most of the supply current drives the
MOSFET gates. In continuous mode operation, this current
is IGATECHG=f(Qg(TOP)+Qg(BOT)).Thejunctiontemperature
can be estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the GN package is
limited to less than 23mA from a 30V supply:
TJ = 70°C + (23mA)(30V)(80°C/W) = 125°C
For applications where more current is needed than INTVCC
can supply, INTVCC can be driver by an external supply
with a voltage higher than 5.35V. However, the INTVCC pin
should not exceed its absolute maximum voltage of 7V.
External Gate Drive Buffers
The LTC3823 drivers are adequate for driving up to about
50nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating
at frequencies requiring greater RMS currents will benet
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 6
can be used.
Soft-Start and Tracking
The LTC3823 has the ability to either soft start by itself with
a capacitor or track the output of another supply. When
the device is congured to soft start by itself, a capacitor
should be connected to the TRACK/SS pin. The LTC3823
is put in a low quiescent current shutdown state (30μA)
if the RUN pin voltage is below 1.5V. The TRACK/SS
pin is actively pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.5V, the LTC3823 is
powered up. A soft-start current of 1.7μA then starts to
charge the soft-start capacitor CSS. Pin Z1/SSENABLE must
be grounded for soft-start operation. Note that soft-start
is achieved not by limiting the maximum output current
of the controller but by controlling the ramp rate of the
output voltage. Current foldback is disabled during this
soft-start phase. During the soft-start phase, the LTC3823
is ramping the reference voltage until it is 20% below the
voltage set by the VREFIN pin. The forced continuous mode
is also disabled and PGOOD signal is forced low during this
phase. The total soft-start time can be calculated as:
tSOFTSTART = 0.5V CSS/1.7μA
When the device is congured to track another supply,
the feedback voltage of the other supply is duplicated by
a resistor divider and applied to the TRACK/SS pin. Pin
Z1/SSENABLE should be tied to INTVCC to turn off the soft-
start current in this mode. Therefore, the voltage ramp rate
on this pin is determined by the ramp rate of the other
supply output voltage.
Output Voltage Tracking
The LTC3823 allows the user to program how its output
ramps up and down by means of the TRACK/SS pin.
Through this pin, the output can be set up to either co-
incidentally or ratiometrically track with another supply’s
output, as shown in Figure 7. In the following discussions,
VOUT1 refers to the master LTC3823’s output and VOUT2
refers to the slave LTC3823’s output.
To implement the coincident tracking in Figure 7a, connect
an additional resistive divider to VOUT1 and connect its
midpoint to the TRACK/SS pin of the slave IC. The ratio of
this divider should be selected the same as that of the slave
IC’s feedback divider shown in Figure 8. In this tracking
APPLICATIONS INFORMATION
Q1
FMMT619
GATE
OF M1
TG
BOOST
SW
Q2
FMMT720
Q3
FMMT619
GATE
OF M2
BG
3823 F06
INTVCC
PGND
Q4
FMMT720
10Ω
Figure 6. Optional External Gate Driver
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