TC105 Datasheet, PDF (6/14 Pages) Microchip Technology – PFM/PWM Step-Down DC/DC Controller

TC105
4.0 APPLICATIONS
4.1 Circuit Examples
Figure 4-3 shows a TC105 using a PNP switching
transistor (Zetex FZT749) that has an h_FE of 180 and
V_CESAT of 100 mV at I_C = 1A. Other high beta transis-
tors can be used, but the values of R_B and C_B may
need adjustment if h_FE is significantly different from that
of the FZT749.
The circuit of Figure 4-4 utilizes a P-channel MOSFET
switching transistor (Silconix Si9430). This transistor is
a member of the Littlefoot™ family of small outline
MOSFETs.
FIGURE 4-1:
SOFT START CIRCUIT
R_SS
470K
V_IN
+ _CSS
4 – 0.033µF
SHDN/SS
TC105
Shutdown Not Used
+
4–
SHDN/SS
TC105
R_SS
470K
C_SS
0.033µF
SHDN
OFF ON
(From System
Control Logic)
FIGURE 4-2:
P
V_IN
Shutdown Used
EXTERNAL TRANSISTOR CONNECTION
L
Q
D
+
–
C_L
V_OUT V_IN
L
D
+
– ^CL
V_OUT
TC105
EXT
1
+
R_{B –} C_B
TC105
EXT
1
P-Channel MOSFET
PNP Bipolar Transistor
DS21349B-page 6
© 2002 Microchip Technology Inc.