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參數(shù)資料
| 型號(hào)： | LTC3547BEDDB |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.7 A DUAL SWITCHING CONTROLLER, 2700 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 2 MM, 0.75 MM HEIGHT, PLASTIC, M0-229WECD-1, DFN-8 |
| 文件頁(yè)數(shù)： | 2/16頁(yè) |
| 文件大?。?/td> | 270K |
| 代理商： | LTC3547BEDDB |
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LTC3547B
10
3547bfa
APPLICATIO S I FOR ATIO
WU
U
Setting the Output Voltage
The LTC3547B regulates the VFB1 and VFB2 pins to 0.6V
during regulation. Thus, the output voltage is set by a
resistive divider according to the following formula:
VV
R
OUT =+
06
1
2
1
4
.( )
Keeping the current small (< 5A) in these resistors maxi-
mizes efciency, but making it too small may allow stray
capacitance to cause noise problems or reduce the phase
margin of the error amp loop.
To improve the frequency response of the main control
loop, a feedback capacitor (CF) may also be used. Great
care should be taken to route the VFB line away from noise
sources, such as the inductor or the SW line.
Fixed output versions of the LTC3547B (e.g. LTC3547B-1)
include an internal resistive divider, eliminating the need
for external resistors. The resistor divider is chosen such
that the VFB input current is 3A. For these versions, the
VFB pin should be connected directly to VOUT. Table 2 lists
the xed output voltages available for the LTC3547B.
Table 2. Fixed Output Voltage Versions
Part Number
VOUT1
VOUT2
LTC3547B
Adjustable
LTC3547B-1
1.8V
1.2V
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD ESR, where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or dis-
charge COUT generating a feedback error signal used by the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot
or ringing that would indicate a stability problem.
The initial output voltage step may not be within the band-
width of the feedback loop, so the standard second-order
overshoot/DC ratio cannot be used to determine the phase
margin. In addition, feedback capacitors (CF1 and CF2)
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor CF provides phase lead by
creating a high frequency zero with R2 which improves
the phase margin.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a re-
view of control loop theory, refer to Application Note 76.
In some applications, a more severe transient can be
caused by switching in loads with large (>1F) input ca-
pacitors. The discharged input capacitors are effectively
put in parallel with COUT, causing a rapid drop in VOUT.
No regulator can deliver enough current to prevent this
problem if the switch connecting the load has low resistance
and is driven quickly. The solution is to limit the turn-on
speed of the load switch driver. A Hot Swap controller
is designed specically for this purpose and usually in-
corporates current limiting, short-circuit protection, and
soft-starting.
Hot Swap is a trademark of Linear Technology Corporation.
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