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| 型号： | LTC3419IMS#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1.2 A DUAL SWITCHING CONTROLLER, 2700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 3/16页 |
| 文件大小： | 266K |
| 代理商： | LTC3419IMS#PBF |
LTC3419
11
3419fa
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
discharge COUT generating a feedback error signal used by
the regulator to return VOUTtoitssteady-statevalue.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
bandwidth 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 (>1μF) input capacitors. 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 incorporates current limiting,
short-circuit protection, and soft-starting.
Efciency Considerations
The percent efciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efciency and which change would
produce the most improvement. Percent efciency can
be expressed as:
% Efciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four sources usually account for the losses in
LTC3419 circuits: 1) VIN quiescent current, 2) switching
losses, 3) I2R losses, 4) other system losses.
1. The VIN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET
driver and control currents. VIN current results in a
small (<0.1%) loss that increases with VIN, even at
no load.
2. The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ moves
from VIN to ground. The resulting dQ/dt is a current
out of VIN that is typically much larger than the DC bias
current. In continuous mode, IGATECHG = fO(QT + QB),
where QT and QB are the gate charges of the internal top
and bottom MOSFET switches. The gate charge losses
are proportional to VIN and thus their effects will be
more pronounced at higher supply voltages.
3. I2R losses are calculated from the DC resistances
of the internal switches, RSW, and external inductor,
RL. In continuous mode, the average output current
ows through inductor L, but is “chopped” between
the internal top and bottom switches. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET RDS(ON) and the duty cycle
(DC) as follows:
RSW = (RDS(ON)TOP) (DC) + (RDS(ON)BOT) (1– DC)
APPLICATIONS INFORMATION
Hot Swap is a trademark of Linear Technology Corporation.
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