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参数资料
| 型号： | LTC3851AEGN#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, PDSO16 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-16 |
| 文件页数： | 8/30页 |
| 文件大小： | 430K |
| 代理商： | LTC3851AEGN#TRPBF |
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LTC3851A
16
3851afa
applicaTions inForMaTion
mode, the TK/SS voltage is substantially higher than
0.8V at steady state and effectively turns off D1. D2 and
D3 will therefore conduct the same current and offer
tight matching between VFB and the internal precision
0.8V reference. In the ratiometric mode, however, TK/SS
equals 0.8V at steady state. D1 will divert part of the bias
current to make VFB slightly lower than 0.8V.
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a finite amount
of output voltage deviation. Furthermore, when the master
supply’s output experiences dynamic excursion (under
load transient, for example), the slave channel output will
be affected as well. For better output regulation, use the
coincident tracking mode instead of ratiometric.
INTVCC Regulator
TheLTC3851AfeaturesaPMOSlowdropoutlinearregulator
(LDO) that supplies power to INTVCC from the VIN supply.
INTVCCpowersthegatedriversandmuchoftheLTC3851A’s
internal circuitry. The LDO regulates the voltage at the
INTVCC pin to 5V.
The LDO can supply a peak current of 50mA and must
be bypassed to ground with a minimum of 2.2μF ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capacitor is used, an additional 0.1μF
ceramic capacitor placed directly adjacent to the INTVCC
and GND pins is highly recommended. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3851A to be
exceeded. The INTVCC current, which is dominated by the
gate charge current, is supplied by the 5V LDO.
Power dissipation for the IC in this case is highest and
is approximately equal to VIN IINTVCC. The gate charge
current is dependent on operating frequency as discussed
in the Efficiency Considerations section. The junction tem-
perature can be estimated by using the equations given in
Note 3 of the Electrical Characteristics. For example, the
LTC3851A INTVCC current is limited to less than 14mA
from a 36V supply in the GN package:
TJ = 70°C + (14mA)(36V)(110°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= INTVCC) at maximum VIN.
Topside MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor, CB, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET.CapacitorCBintheFunctionalDiagramischarged
though external diode DB from INTVCC when the SW pin
is low. When the topside MOSFET is to be turned on, the
driver places the CB voltage across the gate source of the
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage, SW, rises to VIN
R3
VOUT
R4
(4a) Coincident Tracking Setup
TO
VFB
PIN
R3
VMASTER
R4
TO
TK/SS
PIN
R1
R3
VOUT
R4
R2
3851A F04
(4b) Ratiometric Tracking Setup
TO
VFB
PIN
TO
TK/SS
PIN
VMASTER
–
+
I
D1
TK/SS
0.8V
VFB
D2
D3
3851A F05
EA
Figure 4. Setup for Coincident and Ratiometric Tracking
Figure 5. Equivalent Input Circuit of Error Amplifier
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