LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
where L1, L2, etc. are the individual losses as a percentage
of input power. It is often useful to analyze individual
losses to determine what is limiting the efficiency and
which change would produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1625 circuits:
1. INTVCC current. This is the sum of the MOSFET driver
and control currents. The driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched on and then off,
a packet of gate charge Qg moves from INTVCC to
ground. The resulting current out of INTVCC is typically
much larger than the control circuit current. In continu-
ous mode, IGATECHG = f(Qg(TOP) + Qg(BOT)).
By powering EXTVCC from an output-derived source,
the additional VIN current resulting from the driver and
control currents will be scaled by a factor of Duty Cycle/
Efficiency. For example, in a 20V to 5V application at
400mA load, 10mA of INTVCC current results in ap-
proximately 3mA of VIN current. This reduces the loss
from 10% (if the driver was powered directly from VIN)
to about 3%.
2. DC I2R Losses. Since there is no separate sense resis-
tor, DC I2R losses arise only from the resistances of the
MOSFETs and inductor. In continuous mode the aver-
age output current flows through L, but is “chopped”
between the top MOSFET and the bottom MOSFET. If
the two MOSFETs have approximately the same RDS(ON),
then the resistance of one MOSFET can simply be
summed with the resistance of L to obtain the DC I2R
loss. For example, if each RDS(ON) = 0.05Ω and RL =
0.15Ω, then the total resistance is 0.2Ω. This results in
losses ranging from 2% to 8% as the output current
increases from 0.5A to 2A for a 5V output. I2R losses
cause the efficiency to drop at high output currents.
3. Transition losses apply only to the topside MOSFET,
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss = (1.7)(VIN2)(IO(MAX))(CRSS)(f)
4. LTC1625 VIN supply current. The VIN current is the DC
supply current to the controller excluding MOSFET gate
drive current. Total supply current is typically about
850µA. If EXTVCC is connected to 5V, the LTC1625 will
draw only 330µA from VIN and the remaining 520µA will
come from EXTVCC. VIN current results in a small
(< 1%) loss which increases with VIN.
Other losses including CIN and COUT ESR dissipative
losses, Schottky conduction losses during dead time
and inductor core losses, generally account for less
than 2% total additional loss.
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 DC (resistive) load
current. When a load step occurs, VOUT immediately shifts
by an amount equal to (∆ILOAD)(ESR), where ESR is the
effective series resistance of COUT, and COUT begins to
charge or discharge. The regulator loop acts on the
resulting feedback error signal to return VOUT to its steady-
state value. During this recovery time VOUT can be moni-
tored for overshoot or ringing which would indicate a
stability problem. The ITH pin external components shown
in Figure 1 will provide adequate compensation for most
applications.
A second, more severe transient is caused by connecting
loads with large (> 1µF) supply bypass capacitors. The
discharged bypass 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 load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive in order
to limit the inrush current to the load.
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during opera-
tion. But before you connect, be advised: you are plug-
ging into the supply from hell. The main battery line in an
16
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