SC1532 Datasheet, PDF (8/9 Pages) Semtech Corporation – 400mA SmartLDOTM with Internal Pass MOSFET

400mA SmartLDO^TM with Internal Pass
MOSFET
SC1532
January 3, 2000
APPLICATIONS INFORMATION (Cont.)
Inserting VIN1 = 5.5V, VO = 3.234V, I_O = 400mA,
VIN2 = 3.6V, I_Q1 = 4mA and I_Q2 = 2mA yields:
P_{D(MAX )} = 0 .936 W
Using this figure, we can calculate the maximum ther-
mal impedance allowable to maintain T_J ≤ 125°C at an
ambient temperature of 55°C:
( ) _{( ) R = TH(J− A )(MAX )}
T_{J(MAX )} − T_{A (MAX )}
P_{D(MAX )}
=
125 − 55
0.936
= 75°C / W
This is readily achievable using pcb copper area to aid
in conducting the heat away from the device (see Fig-
ure 1 below).
VIN1 Source Impedance
In order to ensure seamless transitions between sup-
plies with VIN1 rising and falling, it is recommended
that the source impedance of VIN1 is less than
0.125Ω. This is because as the output current
switches from VIN1 to VIN2 and visa-versa, the sup-
plies can “chatter” if:
I_O • R _SOURCE > V_HYST
In general, this can be avoided by minimizing supply
trace lengths and resistances. In circumstances where
the source impedance is causing supply “chattering”,
increasing the value of the VIN1 input capacitor should
solve the problem by reducing the instantaneous drop
or jump in VIN1 as the supplies are switched.
Layout Considerations
While layout for linear devices is generally not as criti-
cal as for a switching application, careful attention to
detail will ensure reliable operation. See Figure 1 below
for a sample layout.
1) Attaching the part (pins 5 to 8) to a larger copper
footprint will enable better heat transfer from the de-
vice, especially on PCBs where there are internal
ground and power planes.
2) Place the input and output capacitors close to the
device for optimal transient response.
Top Copper
Top Silk Screen
Fig. 1: Suggested pcb layout based upon Application Circuit on Page 1.
NOTES:
(1) All vias go to ground plane.
(2) Copper area on pins 5 thru 8 is recommended, 0.5” x 0.5” area only is shown. Connect to the ground plane
with a via or vias.
© 2000 SEMTECH CORP.
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