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參數(shù)資料
| 型號： | LTC3419EDD-1#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.2 A DUAL SWITCHING CONTROLLER, 2250 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 3 MM, PLASTIC, MO-229WEED-1, DFN-8 |
| 文件頁數(shù)： | 3/16頁 |
| 文件大?。?/td> | 264K |
| 代理商： | LTC3419EDD-1#TR |
LTC3419
11
3419f
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
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 paral-
lel 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 cur-
rent 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 percent-
age 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 re-
sults 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 cur-
rent. 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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