ltc1875egn-trpbf Linear Technology Corporation, ltc1875egn-trpbf Datasheet - Page 14

• Manufacturer Part Number: ltc1875egn-trpbf
• Description: 15?a Quiescent Current 1.5a Monolithic Synchronous Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1875
To avoid the LTC1875 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. Normally,
some iterative calculation is required to determine a rea-
sonably accurate value. The temperature rise is given by:
where P is the power dissipated by the regulator and
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
where T
transistor R
usually necessary to iterate 2 to 3 times through the
equations to achieve a reasonably accurate value for the
junction temperature.
As an example, consider the LTC1875 in dropout at an
input voltage of 3V, a load current of 0.8A and an ambient
temperature of 70 C. From the typical performance graph
of switch resistance, the R
at 70 C is 0.35 . Therefore, power dissipated by the IC is:
For the SSOP package, the
junction temperature of the regulator is:
However, at this temperature, the R
14
T
T
P = I
T
R
J
J
= T
= 70 C + (0.224)(110) = 95 C
= P •
2
A
A
• R
is the ambient temperature. Because the power
+ T
DS(ON)
JA
DS(ON)
R
= 0.224W
U
is a function of temperature, it is
U
DS(ON)
JA
of the P-channel switch
is 110 C/W. Thus the
DS(ON)
W
is actually 0.4 .
U
JA
Therefore:
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
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
regulator loop then acts to return V
value. During this recovery time, V
for overshoot or ringing that would indicate a stability
problem. The I
tion as shown in Figure 9. (The capacitor, C
needed for 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.
T
J
= 70 C + (0.256)(140) = 98 C
OUT
, causing a rapid drop in V
LOAD
OUT
TH
, generating a feedback error signal. The
OUT
• ESR), where ESR is the effective series
pin can be used for external compensa-
.
OUT
I
LOAD
immediately shifts by an amount
also begins to charge or
OUT
OUT
OUT
to its steady-state
can be monitored
. No regulator can
C2
, is typically
DS(ON)
LOAD
1875f
).
).