SP6120B Datasheet, PDF (19/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

yond the limit set by the 43mV threshold.
A straight forward way to do this would be to
add a resistor R_S2 in parallel with C_S, creating a
voltage divider with R_S. This changes the rela-
tionship with R_S and C_S to be
C_S = 2
L
ESR
1
R_S //R_S2
Using a voltage divider across the inductor, the
new relationship becomes:
I_LIM =
43mV
ESR
R_S + R_S2
R_S2
To calculate R_S2, the formula becomes
/( ^{) RS2 = RS}
I_LIMESR _{– 1}
43mV
SWN
ISP
ISN
R_S2
R_S
C_S
L
ESR
V_OUT
Figure 20: Current Sensing
Output Voltage Programming
As shown in Figure 21(A), the voltage divider
connecting to the V_FB pin programs the output
voltage according to
V_OUT
=
1.25(
1
+
R₁
R₂
)
where 1.25V is the internal reference voltage.
Select R2 in the range of 10k to 100k, and R1 can
be calculated using
R₁
=
R₂(V_OUT –
1.25
1.25)
For output voltage less than 1.25V, a simple
circuit shown in Figure 21(B) can be used in
which V_REF is an external voltage reference
greater than 1.25V. For simplicity, use the same
resistor value for R1 and R2, then R3 is deter-
mined as follows,
R₃
=
(V_REF – 1.25)R₁
2.5 – V_OUT
V_OUT > 1.25V
V_REF
V_OUT < 1.25V
®
R1
SP6120B ^VFB
R2
R3
®
R1
SP6120B
V_FB
R2
A
B
Figures 21(A), a voltage divider connected to the V_FB pin programs the output voltageand 21(B), a simple circuit using
one external voltage reference programs the output voltages to less than 1.25V.
Date: 5/25/04
SP6120B Low Voltage, AnyFET^TM, Synchronous, Buck Controller
19
© Copyright 2004 Sipex Corporation