LTC1877 Linear Technology, LTC1877 Datasheet - Page 12

• Manufacturer Part Number: LTC1877
• Description: High Efficiency Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LTC1877
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:
where P
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T
where T
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 R
P-channel switch at 70 C is approximately 0.9 . There-
fore, power dissipated by the part is:
For the MSOP package, the
junction temperature of the regulator is:
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 (R
12
T
T
P
T
R
J
J
D
= T
= 70 C + (0.225)(150) = 104 C
= (P
= I
D
A
A
LOAD
is the power dissipated by the regulator and
is the ambient temperature.
D
+ T
)(
2
R
JA
• R
)
DS(ON)
U
INFORMATION
= 0.225W
U
J
, is given by:
JA
is 150 C/ W. Thus, the
W
DS(ON)
U
DS(ON)
of the
JA
).
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, V
equal to ( I
resistance of C
discharge C
The regulator loop then acts to return V
state value. During this recovery time V
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 I
external compensation using R
Figure 7. (The 220pF capacitor, C
noise decoupling.)
A second, more severe transient is caused by switching in
loads with large (>1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
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 • C
Thus, a 10 F capacitor charging to 3.3V would require a
250 s 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 (R
C
plate of C
C1
OUT
) and C
, causing a rapid drop in V
LOAD
OUT
IN
C2
, the (–) plate of C
, which generates a feedback error signal.
OUT
. The power ground consists of the (–)
• ESR), where ESR is the effective series
.
OUT
I
LOAD
immediately shifts by an amount
also begins to charge or
C2
TH
, is typically needed for
C
OUT
OUT
, C
pin can be used for
. No regulator can
C1
and Pin 4 of the
OUT
OUT
as shown in
can be moni-
to its steady-
LOAD
C
and
).