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NCP1500DMR2 ,Dual Mode PWM/Linear BUCK ConverterPrepared by: Upal SenguptaON SemiconductorPrincipal Field Applications EngineerAPPLICATION NOTEINTR ..
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NCP1500DMR2-NCP1501DMR2
Dual Mode PWM/Linear BUCK Converter
AND8117/D
Understanding the Output
Current Capability of DC-DC
Buck Converters
Prepared by: Upal SenguptaON Semiconductor
Principal Field Applications Engineer
INTRODUCTIONDue to the widespread availability of highly integrated
DC-DC switchmode converter devices, system design
engineers no longer have to put much effort into the design
of low-power converters for many applications. A little bit
of analysis, however, can allow the system designer to make
sure that an IC switching regulator is being utilized to its full
capability.
Whether a DC-DC converter circuit uses an internal or
external power switch, one critical parameter that the circuit
designer must determine is the load current capability. This
value leads to the sizing of the power switch. The peak
switch current rating (the level of current above which the
power MOS device may break down or overheat) is
proportional to the load current. Of course, a larger power
MOS device will be able to deliver more output current, but
for a given switch current rating, the system designer does
have some ability to affect output current capability based on
external component values and operating conditions as well.
In addition to the peak switch current limit rating, the
effective output current capability for the overall power
supply circuit also depends on the input-output voltage
relationship, the inductor value, and switching frequency.
As a result, system designers need to understand how the
specifications of an IC switching regulator should be
interpreted to apply to their specific operating conditions. In
some cases, the published output current ratings of the IC
may not reflect the true capability of the part for a given
application, whereas in other cases the safe operating limits
of the IC may be inadvertently exceeded.
Buck Converter TopologyA “semi-ideal” synchronous buck converter is illustrated
in Figure 1. The high-side power switch duty-cycle will
depend on the step-down ratio. When the high-side power
switch is turned on, current drawn from the input begins to
flow through
turned off, the low-side (synchronous rectifier) switch is
turned on, and current circulates through the lower NMOS
switch as shown, since the inductor current cannot
instantaneously stop. During steady-state operation, the
“on” and “off” times of the switch are balanced to maintain
the desired output voltage.
IPMOS = Current Flow during TonVin
