Datasheet

14
LTC1875
1875f
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:
T
R
= P • θ
JA
where P is the power dissipated by the regulator and θ
JA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
T
J
= T
A
+ T
R
where T
A
is the ambient temperature. Because the power
transistor R
DS(ON)
is a function of temperature, it is
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
DS(ON)
of the P-channel switch
at 70°C is 0.35Ω. Therefore, power dissipated by the IC is:
P = I
2
• R
DS(ON)
= 0.224W
For the SSOP package, the θ
JA
is 110°C/W. Thus the
junction temperature of the regulator is:
T
J
= 70°C + (0.224)(110) = 95°C
However, at this temperature, the R
DS(ON)
is actually 0.4Ω.
Therefore:
T
J
= 70°C + (0.256)(140) = 98°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 (R
DS(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, V
OUT
immediately shifts by an amount
equal to (∆I
LOAD
• ESR), where ESR is the effective series
resistance of C
OUT
. ∆I
LOAD
also begins to charge or
discharge C
OUT
, generating a feedback error signal. The
regulator loop then acts to return V
OUT
to its steady-state
value. During this recovery time, V
OUT
can be monitored
for overshoot or ringing that would indicate a stability
problem. The I
TH
pin can be used for external compensa-
tion as shown in Figure 9. (The capacitor, C
C2
, 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 C
OUT
, causing a rapid drop in V
OUT
. 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 • C
LOAD
).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
APPLICATIO S I FOR ATIO
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