SP6125 Datasheet, PDF (4/12 Pages) Sipex Corporation – High-Voltage, Step Down Controller in TSOT6

General Overview
The SP6125 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. SP6125 has Type-II internal
compensation for use with Electrolytic or
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 300kHz.
SP6125 contains useful protection features.
Overcurrent 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.
SP6125 Loop Compensation
The SP6125 includes Type-II internal
compensation components for loop compen-
sation. 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.
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)
CZ =
L⋅C
……………………….. (4)
1.3× R1
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.
f_ESRZERO÷f_DBPOLE RZ
1X
50K
2X
40K
3X
30K
5X
10K
>= 10X
2K
Table1- Selection of RZ
Jan28-08 RevG
SP6125: TSOT-6 PFET Buck Controller
4
 2007 Exar Corporation