LTC3406AB Datasheet, PDF (11/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
APPLICATIONS INFORMATION
The junction temperature, T_J, is given by:
T_J = T_A + T_R
where T_A is the ambient temperature.
As an example, consider the LTC3406AB in dropout at
an input voltage of 2.7V, a load current of 600mA and
an ambient temperature of 70°C. From the typical per-
formance graph of switch resistance, the R_DS(ON) of the
P-channel switch at 70°C is approximately 0.27Ω. There-
fore, power dissipated by the part is:
P_D = I_LOAD2 • R_DS(ON) = 97.2mW
For the SOT-23 package, the θ_JA is 250°C/ W. Thus, the
junction temperature of the regulator is:
T_J = 70°C + (0.0972)(250) = 94.3°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature
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 (ΔI_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT, which generates 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. For a detailed explanation of switching control
loop theory, see Application Note 76.
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 paral-
lel 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.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3406AB. These items are also illustrated graphically in
Figures 3 and 4. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace, the V_OUT trace and the V_IN trace should be kept
short, direct and wide.
2. Does the V_FB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be
connected between the (+) plate of C_OUT and ground.
3. Does C_IN connect to V_IN as closely aspossible? This
capacitor provides the AC current to the internal power
MOSFETs.
4. Keep the switching node, SW, away from the sensitive
V_FB node.
5. Keep the (–) plates of C_IN and C_OUT and the IC ground,
as close as possible.
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