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PDF ( 数据手册 , 数据表 ) LM25085A

零件编号 LM25085A
描述 42V Constant On-Time PFET Buck Switching Controller
制造商 National Semiconductor
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LM25085A 数据手册, 描述, 功能
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LM25085A
March 16, 2009
42V Constant On-Time PFET Buck Switching Controller
with 0.9V Reference
General Description
The LM25085A is a functional variant of the LM25085 COT
PFET Buck Switching Controller. The functional differences
of the LM25085A are: The feedback reference voltage is
0.9V, the forced off-time after current limit detection is longer,
and the soft-start time is shorter (1.8 ms).
The LM25085A is a high efficiency PFET switching regulator
controller that can be used to quickly and easily develop a
small, efficient buck regulator for a wide range of applications.
This high voltage controller contains a PFET gate driver and
a high voltage bias regulator which operates over a wide 4.5V
to 42V input range. The constant on-time regulation principle
requires no loop compensation, simplifies circuit implemen-
tation, and results in ultra-fast load transient response. The
operating frequency remains nearly constant with line and
load variations due to the inverse relationship between the
input voltage and the on-time. The PFET architecture allows
100% duty cycle operation for a low dropout voltage. Either
the RDS(ON) of the PFET or an external sense resistor can be
used to sense current for over-current detection.
Features
Wide 4.5V to 42V input voltage range
Adjustable current limit using RDS(ON) or a current sense
resistor
Programmable switching frequency to 1MHz
No loop compensation required
Ultra-Fast transient response
Nearly constant operating frequency with line and load
variations
Adjustable output voltage from 0.9V
Precision ±2% feedback reference
Capable of 100% duty cycle operation
Internal soft-start timer
Integrated high voltage bias regulator
Thermal shutdown
Package
MSOP-8EP
MSOP-8
LLP-8 (3 mm x 3 mm)
Typical Application, Basic Step Down Controller
© 2009 National Semiconductor Corporation 300852
30085201
www.national.com







LM25085A pdf, 数据表
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On-Time vs. Temperature
Minimum On-Time vs. Temperature
30085254
Off-Time vs. Temperature
30085255
Current Limit Comparator Offset vs. Temperature
30085256
ADJ Pin Current vs. Temperature
30085257
PGATE Driver Output Resistance vs. Temperature
www.national.com
30085214
8
30085210







LM25085A equivalent, schematic
CVCC: The capacitor at the VCC pin (from VIN to VCC) pro-
www.DataShveiedte4sUn.cootmonly noise filtering and stability for the VCC regula-
tor, but also provides the surge current for the PFET gate
drive. The typical recommended value for CVCC is 0.47 µF. A
good quality, low ESR, ceramic capacitor is recommended.
CVCC must be located as close as possible to the VIN and
VCC pins. If the selected PFET has a Total Gate Charge
specification of 100 nC or larger, or if the circuit is required to
operate at input voltages below 7 volts, a larger capacitor may
be required. The maximum recommended value for CVCC is
1 µF.
IC Power Dissipation: The maximum power dissipated in the
LM25085A package is calculated using Equation 12 at the
maximum input voltage. The Total Gate Charge for the
Si7465 PFET is specified to be 40 nC (max) in its data sheet.
Therefore the total power dissipation within the LM25085A is
calculated to be:
PDISS = 24V x ((40 nC x 200 kHz) + 1.25 mA) = 222 mW
Using an MSOP-8EP package with a θJA of 46°C/W produces
a temperature rise of 10°C from junction to ambient.
Final Design Example Circuit
The final circuit is shown in Figure 4, and its performance is
presented in Figure 5 through Figure 8. The measured effi-
ciencies shown in Figure 5 are typical for a buck converter
producing a low output voltage (1V).
FIGURE 4. Example Circuit
30085242
www.national.com
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