Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- OPERATING RANGE
- ELECTRICAL CHARACTERISTICS
- TYPICAL PERFORMANCE CHARACTERISTICS
- BLOCK DIAGRAM
- APPLICATION INFORMATION
- CONSTANT ON-TIME CONTROL OVERVIEW
- INTERNAL OPERATION UNDER-VOLTAGE COMPARATOR
- ON-TIME GENERATOR SHUTDOWN
- CURRENT LIMIT
- N-CHANNEL HIGH SIDE SWITCH AND DRIVER
- THERMAL SHUTDOWN
- COMPONENT SELECTION
- FREQUENCY SELECTION
- INDUCTOR SELECTION
- OUTPUT CAPACITOR
- RIPPLE FEED FORWARD
- FEEDBACK RESISTORS
- INPUT CAPACITOR
- AVIN CAPACITOR
- SOFT-START CAPACITOR
- EXTVCC CAPACITOR
- SHUTDOWN
- CBOOT CAPACITOR
- PGOOD RESISTOR
- CATCH DIODE
- BYPASS CAPACITOR
- EXTERNAL OPERATION STARTUP
- UNDER- & OVER-VOLTAGE CONDITIONS
- CURRENT LIMIT
- MODES OF OPERATION
- LINE REGULATION
- TRANSIENT RESPONSE
- EFFICIENCY
- PRE-BIAS LOAD STARTUP
- THERMAL CONSIDERATIONS
- LAYOUT CONSIDERATIONS
- Revision History
C
ff_MAX
=
(V
IN_MIN
- V
FB
) x T
ON_MIN
0.03V x R
ff
pF
R
ff
R
FB1
R
FB2
C
ff
SW
FB
L
C
OUT
V
OUT
H =
V
OUT
V
FB
=
V
OUT
1.25V
LM2696
www.ti.com
SNVS375B –OCTOBER 2005–REVISED APRIL 2013
To ensure stability, two constraints need to be met. These constraints are the voltage ripple at the feedback pin
must be greater than some minimum value and the voltage ripple must be in phase with the switch pin.
The ripple voltage necessary at the feedback pin may be estimated using the following relationship:
ΔV
FB
> −0.057 • f
SW
+ 35
where
• f
SW
is in kHz
• ΔV
FB
is in mV. (9)
This minimum ripple voltage is necessary in order for the comparator to initiate switching. The voltage ripple at
the feedback pin must be in-phase with the switch. Because the ripple due to the capacitor charging and
capacitor ESR are out of phase, the ripple due to capacitor ESR must dominate.
The ripple at the output may be calculated by multiplying the feedback ripple voltage by the gain seen through
the feedback resistors. This gain H may be expressed as:
(10)
To simplify design and eliminate the need for high ESR output capacitors, an RC network may be used to feed
forward a signal from the switchpin to the feedback (FB) pin. See the ‘RIPPLE FEED FORWARD’ section for
more details.
Typically, the best performance is obtained using POSCAPs, SP CAPs, tantalum, Niobium Oxide, or similar
chemistry type capacitors. Low ESR ceramic capacitors may be used in conjunction with the RC feed forward
scheme; however, the feed forward voltage at the feedback pin must be greater than 30 mV.
RIPPLE FEED FORWARD
An RC network may be used to eliminate the need for high ESR capacitors. Such a network is connected as
shown in Figure 18.
Figure 18. RC Feed Forward Network
The value of R
ff
should be large in order to prevent any potential offset in V
OUT
. Typically the value of R
ff
is on the
order of 1 MΩ and the value of R
FB1
should be less than 10 kΩ. The large difference in resistor values minimizes
output voltage offset errors in DCM. The value of the capacitor may be selected using the following relationship:
where
• on-time (T
ON_MIN
) is in µs
• resistance (R
ff
) is in MΩ. (11)
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