ELM605DA
2A synchronous step-down DC/DC converter
8. Power good
ELM605DA also includes an open-drain power good output that indicates when the regulator output is over
90%of its nominal output. If the output voltage is beyond this range, the power good output is pulled to ground.
Since this comparator has no hysteresis on either threshold, a 30μs delay time is added to prevent the power
good output from chattering between states. The power good should be pulled to VIN or another supply voltage
less than 5.5V through a resistor.
■Application notes
1.Input Capacitor Selection
It is necessary for the input capacitor to sustain the ripple current produced during the period of “on” state of
the upper MOSFET, so a low ESR is required to minimize the loss. The RMS value of this ripple can be ob-
tained by the following:
IinRMS = Iout √ D × ( 1 - D )
where D is the duty cycle, IinRMS is the input RMS current, and Iout is the load current. The equation reaches
its maximum value with D = 0.5. The loss of the input capacitor can be calculated by the following equation:
Pcin = ESRcin × IinRMS2
where Pcin is the power loss of the input capacitor and ESRcin is the effective series resistance of the input ca-
pacitance. Due to large dI/dt through the input capacitor, electrolytic or ceramics should be used. If a tantalum is
required, it must be surge-protected. Otherwise, capacitor failure could occur.
2. Output inductor selection
The output inductor selection is to meet the requirements of the output voltage ripple and affects the load tran-
sient response. The higher inductance can reduce the inductor’s ripple current and induce the lower output ripple
voltage. The ripple voltage and current are approximated by the following equations:
Vin - Vout
Fs × L
Vout
Vin
∆I =
×
∆Vout = ∆I × ESR
Although the increase of the inductance reduces the ripple current and voltage, it contributes to the increase of
the response time for the regulator to load transient as well. Increasing the switching frequency (Fs) for a given
inductor also can reduce the ripple current and voltage but it will increase the switching loss of the power MOS.
The way to set a proper inductor value is to have the ripple current (∆I) be approximately 10%~50% of the
maximum output current. Once the value has been determined, select an inductor capable of carrying the re-
quired peak current without going into saturation. It is also important to have the inductance tolerance specified
to keep the accuracy of the system controlled. Using 20% for the inductance (at room temperature) is reasonable
tolerance able to be met by most manufacturers. For some types of inductors, especially those with core made of
ferrite, the ripple current will increase abruptly when it saturates, which will result in a larger output ripple volt-
age.
3. Output capacitor selection
An output capacitor is required to filter the output and supply the load transient current. The high capacitor val-
ue and low ESR will reduce the output ripple and the load transient drop. These requirements are met by a mix
of capacitors and careful layout.
Rev.1.2
13 - 5
ELM-TECH [ ELM Technology Corporation ]
