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| 型号： | ADP1877HC-EVALZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 18/32页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION 13A ADP1877 |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 2，非隔离 |
| 输出电压： | 1.05V，1.8V |
| 电流 - 输出： | 13A |
| 输入电压： | 6 ~ 14 V |
| 稳压器拓扑结构： | 降压 |
| 频率 - 开关： | 600kHz |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | ADP1877 |
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ADP1877
ACCURATE CURRENT - LIMIT SENSING
R DSON of the MOSFET can vary by more than 50% over the
temperature range. Accurate current limit sensing can be
R RAMP
VIN
Data Sheet
achieved by adding a current sense resistor from the source of
the low-side MOSFET to PGND. Make sure that the power
RAMP
DHx
rating of the current sense resistor is adequate for the application.
Apply the above equation and calculate R ILIM by replacing
R DSON_MAX with R SENSE . The Figure 31 illustrates the
implementation of this accurate current limit sensing.
VIN
SWx
ILIMx
DLx
ADP1877
R ILIM
R CSG
ADP1877
DHx
SWx
Figure 32. Slope Compensation and CS Gain Connection
SETTING THE CURRENT SENSE GAIN
ILIMx
DLx
R ILIM
The voltage drop across the external low-side MOSFET is
sensed by a current sense amplifier by multiplying the peak
inductor current and the R DSON of the MOSFET. The result is
R SENSE
Figure 31. Accurate Current Limit Sensing
SETTING THE SLOPE COMPENSATION
In a current-mode control topology, slope compensation is
needed to prevent subharmonic oscillations in the inductor
current and to maintain a stable output. The external slope
compensation is implemented by summing the amplified sense
signal and a scaled voltage at the RAMPx pin. To implement the
slope compensation, connect a resistor between RAMPx and
the input voltage. The resistor, R RAMP , is calculated by
then amplified by a gain factor of either 3 V/V, 6 V/V, 12 V/V,
or 24 V/V, which is programmable by an external resistor, R CSG ,
connected to the DL pin. This gain is sensed only during
power-up and not during normal operation. The amplified
voltage is summed with the slope compensation ramp voltage
and fed into the PWM controller for a stable regulation voltage.
The voltage range of the internal node, V CS , is between 0.4 V
and 2.2 V. Select the current sense gain such that the internal
minimum amplified voltage (V CSMIN ) is above 0.4 V and the
maximum amplified voltage (V CSMAX ) is 2.1 V. Do not set V CSMAX
above 2.1 V to account for temperature and part-to-part
R RAMP =
3 . 6 × 10 10 L
A CS × R DSON _ MAX
variations. Note that V CSMIN or V CSMAX is not the same as V COMP ,
which has a range of 0.75 V to 2.25 V. The following are
equations for V CSMIN and V CSMAX :
V CSMIN = 0 . 75 V ?
I LPP × R DSON _ MIN × A CS
V CSMAX = 0 . 75 V + ( I LOADMAX ? I LPP ) × R DSON _ MAX × A CS
V IN ? 0 . 2 V
where:
3.6 × 10 10 is an internal parameter.
L is the inductance of the inductor.
R DSON_MAX is the the low-side MOSFET maximum on resistance.
A CS is the gain, either 3 V/V, 6 V/V, 12 V/V, or 24 V/V, of the
section for more details).
Keep in mind that R DSON is temperature dependent and can vary
as much as 0.4%/ o C. Choose R DSON at the maximum operating
temperature. The voltage at RAMPx is fixed at 0.2 V, and the
current going into RAMPx should be in between 6 μA and
200 μA. Make sure that the following condition is satisfied:
6 μA ≤ ≤ 200 μA
R RAMP
For instance, with an input voltage of 12 V, R RAMP should not
exceed 1.9 MΩ. If the calculated R RAMP produces less than 6 μA,
then select a R RAMP value that produces between 6 μA and 20 μA.
resistor R RAMP and the current sense gain resistor R CSG .
1
2
1
2
where:
V CSMIN is the minimum amplified voltage of the internal current
sense amplifier at zero output current.
V CSMAX is the maximum amplified voltage of the internal current
sense amplifier at maximum output current.
R DSON_MIN is the the low-side MOSFET minimum on resistance.
The zero-current level voltage of the current sense amplifier is
0.75 V.
I LPP is the peak-to-peak ripple current in the inductor.
I LOADMAX is the maximum output DC load current.
given R DSON maximum load current and a 33% inductor current
ripple. Because of the variation in R DSON of the power MOSFETs
(part-to-part variation and overtemperature) and the variation
of the inductors, the users must verify that V COMP does not
exceed 2.2 V at the maximum output load current.
Rev. D | Page 18 of 32
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