SP6138 Datasheet, PDF (8/17 Pages) Sipex Corporation – Synchronous Buck Controller

APPLICATION INFORMATION
Increasing the Current Limit
If it is desired to set Imax > {60mV / DCR}
(in this case larger than 14.6A), then a resis-
tor R_S3 should be added as shown in figure
2. R_S3 forms a resistor divider and reduces
the voltage seen by the comparator.
Since: 60mV
( Imax • DCR )
R_S3 ⁼ {R_S1 + R_S2 + R_S3}
Solving for RS3 we get:
R_S3 = ^{[60mV • (RS1 + RS2)] .........(2a)}
[(Imax • DCR) – 60mV]
Combining the above equations and solv-
ing for R_S3:
R_S3 =
R_S2 • [VOUT - 60mV + (IMAX•DCR)].........(2b)
60mV - (IMAX • DCR)
As an example: for Imax of 12A and Vout of 3.3V,
calculated R_S3 is 1.5MΩ (232KΩ standard).
SP613X
SWN
L = 2.7uH, DCR = 4.mOhm
Vout
RS
5.K
RS2
5.K
As an example: if desired Imax is 17A, then
R_S3 = 63.4KΩ.
SP613X
SWN
L = 2.7uH, DCR = 4.mOhm
Vout
RS
5.K
RS2
5.K
RS3
63.4K
ISP
CSP
ISN
6.8nF
CS
0.uF
Figure 2- Over-current detection circuit
for Imax > 60mV / DCR
Decreasing the Current Limit
If it is required to set Imax < {60mV / DCR}, a
resistor is added as shown in figure 3. RS3
increases the net voltage detected by the
current-sense comparator. Voltage at the
positive and negative terminal of compara-
tor is given by:
VSP = Vout + (Imax • DCR)
VSN = Vout x {R_S3 / (R_S2 +R_S3)}
Since the comparator is triggered at 60mV:
VSP-VSN = 60 mV
ISP
CSP
CS
ISN
6.8nF
0.uF
RS3
.5MOhm
Figure 3- Over-current detection circuit
for Imax < {60mV / DCR}
Power MOSFET Selection
There are four main criterion in selecting
Power MOSFETs for buck conversion:
● _{Voltage rating BVdss}
● _{On resistance Rds(on)}
● _{Gate-to-drain charge Qgd}
● _{Package type}
In order to better illustrate the MOSFET se-
lection process, the following buck converter
design example will be used: Vin = 12V, Vout
= 3.3V, Iout = 10A, f = 2000KHz, DCR =
4.5mΩ (inductor DC resistance), efficiency
= 94% and Ta = 40˚C.
Select the voltage rating based on maximum
input voltage of the converter. A commonly
used practice is to specify BVdss at least
twice the maximum converter input voltage.
This is done to safeguard against switching
transients that may break down the MOSFET.
For converters with Vin of less than 10V, a
Oct 24-06 Rev J
SP6138 Synchronous Buck Controller

© 2006 Sipex Corporation