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| 型号： | ADP1877HC-EVALZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 23/32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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Data Sheet
SWITCHING NOISE AND OVERSHOOT REDUCTION
In any high speed step-down regulator, high frequency noise
(generally in the range of 50 MHz to 100 MHz) and voltage
overshoot are always present at the gate, the switch node (SW),
and the drains of the external MOSFETs. The high frequency
noise and overshoot are caused by the parasitic capacitance,
C GD , of the external MOSFET and the parasitic inductance of
the gate trace and the packages of the MOSFETs. When the high
current is switched, electromagnetic interference (EMI) is
generated, which can affect the operation of the surrounding
circuits. To reduce voltage ringing and noise, it is required to
add an RC snubber between SW and PGND for applications with
Snubbers may also be needed in applications where the duty
cycle in one of the channels is higher than or equal to 50%.
In most applications, R SNUB is typically 2 Ω to 4 Ω, and C SNUB
typically 1.2 nF to 3 nF.
R SNUB can be estimated by
ADP1877
VOLTAGE TRACKING
The ADP1877 includes a tracking feature that tracks a master
voltage. This feature is especially important when the ADP1877
is powering separate power supply voltages on a single integrated
circuit, such as the core and I/O voltages of a DSP or microcon-
troller. In these cases, improper sequencing can cause damage
to the load.
In all tracking configurations, the output can be set as low as 0.6 V
for a given operating condition. The soft start time setting of
the master voltage should be longer than the soft start of the
slave voltage. This forces the rise time of the master voltage to
be imposed on the slave voltage. If the soft start setting of the
slave voltage is longer, the slave comes up more slowly, and the
tracking relationship is not seen at the output.
coincident and ratiometric trackings. Full time DDR termination is
not recommended when using these tracking features.
COINCIDENT TRACKING
R SNUB ? 2
L MOSFET
C OSS
The most common application is coincident tracking, used in
core vs. I/O voltage sequencing and similar applications.
And C SNUB can be estimated by
C SNUB ? C OSS
where :
L MOSFET is the total parasitic inductance of the high-side and
low-side MOSFETs, typically 3 nH, and is package dependent.
C OSS is the total output capacitance of the high-side and low-
side MOSFETs given in the MOSFET data sheet.
The size of the RC snubber components need to be chosen
correctly to handle the power dissipation. The power dissipated
in R SNUB is
P SNUB = V IN 2 × C SNUB × f SW
In most applications, a component size 0805 for R SNUB is sufficient.
Coincident tracking limits the slave output voltage to be the
same as the master voltage until it reaches regulation. Connect
the slave TRK input to a resistor divider from the master voltage
that is the same as the divider used on the slave FB pin. This
forces the slave voltage to be the same as the master voltage. For
coincident tracking, use R TRKT = R TOP and R TRKB = R BOT , as shown
MASTER VOLTAGE
SLAVE VOLTAGE
TIME
Figure 36. Coincident Tracking
However, the use of an RC snubber reduces the overall efficiency,
generally by an amount in the range of 0.1% to 0.5%. The RC
snubber cannot reduce the voltage overshoot. A resistor, shown
as R RISE in Figure 35, at the BSTx pin helps to reduce overshoot
and is generally between 2 Ω and 4 Ω. Adding a resistor in
VCCO
EN
1 M ?
EN1
TRK1
EN2
FB1
45.3k ?
10k ?
3.3V
V OUT1_MASTER
R TRKT
20k ?
1.1V
R TRKB
10k ?
series, typically between 2 Ω and 4 Ω, with the gate driver also
ADP1877
R RISE
helps to reduce overshoot. If a gate resistor is added, then R RISE
is not needed.
VIN
VDL
BST1
C SS1
100nF
SS1
FB2
TRK2
SS2
C SS2
20nF
DH1
M1
L
R BOT
10k ?
R TOP
20k ?
1.8V
V OUT2_SLAVE
SW1
ILIM1
DL1
R ILIM1
M2
R SNUB
C SNUB
VOUT
COUTx
Figure 37. Example of a Coincident Tracking Circuit
PGND1
ADP1877
(CHANNEL 1)
Figure 35. Application Circuit with a Snubber
Rev. D | Page 23 of 32
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