SP6126 Datasheet, PDF (4/14 Pages) Sipex Corporation – High-Voltage, Step Down Controller in TSOT6

The SP6126 is a fixed frequency, voltage-
mode, non-synchronous PWM controller
optimized for minimum component, small form
factor and cost effectiveness. It has been
designed for single-supply operation ranging
from 4.5V to 29V. SP6126 has Type-II internal
compensation
for
use
with
Electrolytic/Tantalum output capacitors. For
ceramic capacitors Type-III compensation can
be implemented by simply adding an R and C
between output and Feedback. A precision
0.6V reference, present on the positive terminal
of the Error Amplifier, permits programming of
the output voltage down to 0.6V via the FB pin.
The output of the Error Amplifier is internally
compared to a feed-forward (VIN/5 peak-to-
peak) ramp and generates the PWM control.
Timing is governed by an internal oscillator that
sets the PWM frequency at 600kHz.
SP6126 contains useful protection features.
Over-current protection is based on high-side
MOSFET’s Rds(on) and is programmable via a
resistor placed at LX node. Under-Voltage
Lock-Out (UVLO) ensures that the controller
starts functioning only when sufficient voltage
exists for powering IC’s internal circuitry.
SP6126 Loop Compensation
The SP6126 includes Type-II internal
compensation components for loop
compensation.
External compensation
components are not required for systems with
tantalum or aluminum electrolytic output
capacitors with sufficiently high ESR. Use the
condition below as a guideline to determine
whether or not the internal compensation is
sufficient for your design.
GENERAL OVERVIEW
Creating a Type-III compensation Network
The above condition requires the ESR zero to
be at a lower frequency than the double-pole
from the LC filter. If this condition is not met,
Type-III compensation should be used and can
be accomplished by placing a series RC
combination in parallel with R1 as shown
below. The value of CZ can be calculated as
follows and RZ selected from table 1.
CZ =
L⋅C
………….. (4)
R1
f f _ESRZERO ÷ _DBPOLE
1X
2X
3X
5X
>= 10X
RZ
50KΩ
40KΩ
30KΩ
10KΩ
2KΩ
Table1- Selection of RZ
SP6126
CP1 2pF
Vout
RZ
Type-II internal compensation is sufficient if the
following condition is met:
f < f _ESRZERO
DBPOLE
………………. (1)
where:
f _ESRZERO
=
1
2.π .R_ESR .C_OUT
……….. (2)
f _DBPOLE = _{2.π .}
1
L ⋅ C_OUT
………… (3)
CZ2 130pF
RZ2 200k
Vref =0.6V
Error Amplif ier
VFB
CZ
R1
200k, 1%
R2
Figure 1- RZ and CZ in conjunction with
internal compensation components form
a Type-III compensation
Nov07-08 RevG
SP6126: TSOT-6 PFET Buck Controller
4
 2008 Exar Corporation