SP6127 Datasheet, PDF (4/12 Pages) Sipex Corporation – High-Voltage, Step-Down Controller in TSOT6

____________________________________________________ GENERAL OVERVIEW
The SP6127 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.
SP6127 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 900kHz.
SP6127 contains useful protection features.
Overcurrent protection is based on the 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.
SP6127 Loop Compensation
The SP6127 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.
Type-II internal compensation is sufficient if the
following condition is met:
f < f _ESRZERO
DBPOLE
………………. (1)
where:
1
f _ESRZERO = _{2.π .RESR .COUT} ……….. (2)
1
f _DBPOLE = _{2.π . L ⋅ COUT} ………… (3)
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)
1.25 × R1
f_ESRZERO/
f_DBPOLE
1X
2X
3X
5X
>= 10X
RZ
50KΩ
40KΩ
30KΩ
10KΩ
2KΩ
Table1- Selection of RZ
June 26, 2007
SP6127 TSOT-6 PFET Buck Controller
Page 4
 2007 Sipex Corporation