LM5019 Datasheet, PDF (13/17 Pages) Texas Instruments – 100V, 100mA Constant On-Time Synchronous Buck Regulator

APPLICATION CIRCUIT: 12V TO 95V INPUT AND 10V,
100mA OUTPUT BUCK CONVERTER
The application schematic of a buck supply is shown in Figure
6 below. For output voltage (V_OUT) above the maximum reg-
ulation threshold of V_CC (8.3V, see electrical characteristics),
the V_CC pin can be connected to V_OUT through a diode (D2),
as shown below, for higher efficiency and lower power dissi-
pation in the IC.
RIPPLE CONFIGURATION
LM5019 uses Constant-On-Time (COT) control scheme, in
which the on-time is terminated by an on-timer, and the off-
time is terminated by the feedback voltage (V_FB) falling below
the reference voltage (V_REF). Therefore, for stable operation,
the feedback voltage must decrease monotonically, in phase
with the inductor current during the off-time. Furthermore, this
change in feedback voltage (V_FB) during off-time must be
large enough to suppress any noise component present at the
feedback node.
Table 1 shows three different methods for generating appro-
priate voltage ripple at the feedback node. Type 1 and Type
2 ripple circuits couple the ripple at the output of the converter
to the feedback node (FB). The output voltage ripple has two
components:
1. Capacitive ripple caused by the inductor current ripple
charging/discharging the output capacitor.
2. Resistive ripple caused by the inductor current ripple
flowing through the ESR of the output capacitor.
The capacitive ripple is not in phase with the inductor current.
As a result, the capacitive ripple does not decrease mono-
tonically during the off-time. The resistive ripple is in phase
with the inductor current and decreases monotonically during
the off-time. The resistive ripple must exceed the capacitive
ripple at the output node (V_OUT) for stable operation. If this
condition is not satisfied unstable switching behavior is ob-
served in COT converters, with multiple on-time bursts in
close succession followed by a long off-time.
Type 3 ripple method uses R_r and C_r and the switch node
(SW) voltage to generate a triangular ramp. This triangular
ramp is ac coupled using C_ac to the feedback node (FB). Since
this circuit does not use the output voltage ripple, it is ideally
suited for applications where low output voltage ripple is re-
quired. See application note AN-1481 for more details for
each ripple generation method.
30181733
FIGURE 6. Final Schematic for 12V to 95V Input, and 10V, 100mA Output Buck Converter
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