AMS4123
3A 20V Step-Down Converter + 1A LDO
Device Summary
Fault Protection
The AMS4123 is combines a high voltage 3 Amp fixed
frequency step-down converter combined with a 1
Amp low drop out (LDO) linear regulator on a single
die.
Short circuit and over-temperature shutdown disable
the converter and LDO in the event of an overload
condition.
The peak current mode step-down converter has
internal compensation and is stable with a wide range
of ceramic, tantalum, and electrolytic output
capacitors. The step-down converter output voltage is
sensed through an external resistive divider that feeds
the negative input to an internal transconductance
error amplifier. The output of the error amplifier is
connected to the input to a peak current mode
comparator. The inductor current is sensed as it
passes through the power switch, amplified and is
also fed to the current mode comparator. The error
amplifier regulates the output voltage by controlling
the peak inductor current passing through the power
switch so that, in steady state, the average inductor
current equals the load current. The step-down
converter has an input voltage range of 4.5V to 20V
with an output voltage as low as 0.6V.
The LDO operates from an input voltage ranging from
1V to 20V and a typical dropout voltage of 350mV at
1A. The input to the LDO can be supplied by the
output of the Step-Down converter or some other
available power source that must be 2V less than the
input voltage (Vin). The LDO is also stable for a wide
range of ceramic output capacitors ranging from as
low as 1µF.
Application
Inductor
The step-down converter inductor is typically selected
to limit the ripple current to 40% of the full load output
current. Solve for this value at the maximum input
voltage where the inductor ripple current is greatest.
Vo
ꢀ
ꢁ
L= Vin-Vo ·
Vin·Io·0.4·Fs
2.5V
ꢀ
ꢁ
L= 15V-2.5V ·
=9.4µH
15V·2A·0.4·300kHz
For most applications the duty cycle of the AMS4123
step down converter is less than 50% duty and does
not require slope compensation for stability. This
provides some flexibility in the selected inductor
value. Given the above selected value, others values
slightly greater or less may be examined to determine
the effect on efficiency without a detrimental effect on
stability.
With and inductor value selected, the ripple current
can be calculated:
(Vo+Vfwd)·(1-D)
Ipp=
L·Fs
Enable
The enable input has two levels so that the step-down
converter can be enabled independently of the LDO.
The enable threshold for the step-down converter is
2.0V while the enable threshold for the linear regulator
output is 2.5V typical.
Using the maximum input voltage values the ripple is:
ꢀ
ꢁ
(2.5V+0.2V)· 1-0.23
Ipp=
=0.7A
10μH·300kHz
Under Voltage Lockout
Once the appropriate value is determined, the
component is selected based on the DC current and
the peak (saturation) current. Select an inductor that
has a DC current rating greater than the full load
current of the application. The DC current rating is
also reflected in the DC resistance (DCR)
specification of the inductor. The inductor DCR should
limit the inductor loss to less than 2% of the step-
down converter output power.
The under-voltage lockout (UVLO) feature guarantees
sufficient input voltage (Vin) bias for proper operation
of all internal circuitry prior to activation. The input
voltage (Vin) is internally monitored and the converter
and LDO are enabled when the rising level of Vin
reaches 4.2V. To prevent UVLO chatter 400mV of
hysteresis is built in to the UVLO comparator so that
the step-down converter and LDO are disabled when
VIN drops to 3.8V.
The peak current at full load is equal to the full load
DC current plus one half of the ripple current. As
mentioned before, the ripple current varies with input
3/5/2010
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