LTC1877 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1877' PDF : 16 Pages View PDF
LTC1877
APPLICATIONS INFORMATION
Thermal Considerations
In most applications the LTC1877 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC1877 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such as
in dropout, the heat dissipated may exceed the maximum
junction temperature of the part. If the junction tempera-
ture reaches approximately 150°C, both power switches
will be turned off and the SW node will become high
impedance.
To avoid the LTC1877 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
TR = (PD)(θJA)
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC1877 in dropout at an
input voltage of 3V, a load current of 500mA, and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.9Ω. There-
fore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 0.225W
For the MSOP package, the θJA is 150°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.225)(150) = 104°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (RDS(ON)).
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 (∆ILOAD • ESR), where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT, which generates a feedback error signal.
The regulator loop then acts to return VOUT to its steady-
state value. During this recovery time VOUT can be moni-
tored for overshoot or ringing that would indicate a stabil-
ity problem. The internal compensation provides adequate
compensation for most applications. But if additional
compensation is required, the ITH pin can be used for
external compensation using RC, CC1 as shown in
Figure 7. (The 220pF capacitor, CC2, is typically needed for
noise decoupling.)
A second, more severe transient is caused by switching in
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 so that
the load rise time is limited to approximately (25 • CLOAD).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1877. These items are also illustrated graphically in
the layout diagram of Figure 7. Check the following in your
layout:
1. Are the signal and power grounds segregated? The
LTC1877 signal ground consists of the resistive
divider, the optional compensation network (RC and
CC1) and CC2. The power ground consists of the (–)
plate of CIN, the (–) plate of COUT and Pin 4 of the
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