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

L =
(V
IN
- V
OUT
) D
0.3 f
SW
I
OUT
'I
L
=
(V
IN
- V
OUT
) D
L f
SW
f
SW
=
D
T
ON
10
3
kHz
R
ON
=
(V
IN
- V
D
) ‡'
10
6
k
ON
‡I
SW
k
T
ON
=
k
ON
R
ON
V
IN
- V
D
10
-3
Ps
LM2696
SNVS375B –OCTOBER 2005–REVISED APRIL 2013
www.ti.com
The switching frequency of the LM2696 is set by the resistor connected to the RON pin. This resistor controls the
current flowing into the RON pin and is directly related to the on-time pulse. Connecting a resistor from this pin to
PVIN allows the switching frequency to remain constant as the input voltage changes. In normal operation this
pin is approximately 0.65V above GND. In shutdown, this pin becomes a high impedance node to prevent current
flow.
The ON time may be expressed as:
where
• V
IN
is the voltage at the high side of the R
ON
resistor (typically PV
IN
)
• V
D
is the diode voltage present at the RON pin (0.65V typical)
• R
ON
is in kΩ
• k
ON
is a constant value set internally (66 µA•µs nominal). (3)
This equation can be re-arranged such that R
ON
is a function of switching frequency:
where
• f
SW
is in kHz. (4)
In CCM the frequency may be determined using the relationship:
(5)
(T
ON
is in µs)
Which is typically used to set the switching frequency.
Under no condition should a bypass capacitor be connected to the R
ON
pin. Doing so couples any AC
perturbations into the pin and prevents proper operation.
INDUCTOR SELECTION
Selecting an inductor is a process that may require several iterations. The reason for this is that the size of the
inductor influences the amount of ripple present at the output that is critical to the stability of an adaptive on-time
circuit. Typically, an inductor is selected such that the maximum peak-to-peak ripple current is equal to 30% of
the maximum load current. The inductor current ripple (ΔI
L
) may be expressed as:
(6)
Therefore, L can be initially set to the following by applying the 30% guideline:
(7)
The other features of the inductor that should be taken into account are saturation current and core material. A
shielded inductor or low profile unshielded inductor is recommended to reduce EMI.
OUTPUT CAPACITOR
The output capacitor size and ESR have a direct affect on the stability of the loop. This is because the adaptive
on-time control scheme works by sensing the output voltage ripple and switching appropriately. The output
voltage ripple on a buck converter can be approximated by assuming that the AC inductor ripple current flows
entirely into the output capacitor and the ESR of the capacitor creates the voltage ripple. This is expressed as:
ΔV
OUT
≈ ΔI
L
• R
ESR
(8)
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