Rainbow Electronics MAX15023 - Manual (Page 18)

MAX15023
Wide 4.5V to 28V Input, Dual-Output
Synchronous Buck Controller
18 ______________________________________________________________________________________
Setting the Cycle-by-Cycle, Low-Side,
Source Peak Current Limit
The minimum current-limit threshold must be high
enough to support the maximum expected load current
with the worst-case low-side MOSFET on-resistance
value since the low-side MOSFET’s on-resistance is
used as the current-sense element. The inductor’s
cycle-by-cycle, low-side, source peak current occurs at
I
LOAD(MAX) minus half the ripple current. The ripple cur-
rent is maximum when the inductor value is at the lower
limit of its specified tolerance. The minimum value of
the current-limit threshold voltage (V
ITH ) should be
greater than the voltage on the low-side MOSFET dur-
ing the ripple-current valley:
where R DS(ON) is the on-resistance of the low-side
MOSFET in ohms. Use the maximum value for R DS(ON)
from the low-side MOSFET’s data sheet.
To adjust the current-limit threshold, connect a resistor
(R
LIM_ ) from LIM_ to SGND. The relationship between
the current-limit threshold (V ITH_ ) and R LIM_ is:
where R LIM_ is in k
Ω and V
ITH_ is in mV.
An R LIM_ resistance range of 6k
Ω to 60kΩ corresponds
to a current-limit threshold of 30mV to 300mV. When
adjusting the current limit, use 1% tolerance resistors to
minimize errors in the current-limit threshold setting.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The two converters of the MAX15023 run 180° out-of-
phase, thereby, effectively doubling the switching fre-
quency at the input and lowering the input RMS current.
The input ripple waveform would be unsymmetrical due
to the difference in load current and duty cycle between
converter 1 and converter 2. In fact, the worst-case input
RMS current occurs when only one controller is operat-
ing. The converter delivering the highest output power
(V
OUT
x I
OUT
) must be used in the formulas below:
The input capacitor RMS current requirement (I RMS ) is
defined by the following equation:
I RMS has a maximum value when the input voltage
equals twice the output voltage (V IN = 2V OUT ), so
I RMS(MAX) = I LOAD(MAX) /2.
Choose an input capacitor that exhibits less than +10°C
temperature rise at the RMS input current for optimal
long-term reliability.
The input voltage ripple is composed of
∆V
Q
(caused
by the capacitor discharge) and
∆V
ESR
(caused by the
ESR of the capacitor). Use low-ESR ceramic capacitors
with high ripple current capability at the input. Assume
the contribution from the ESR and capacitor discharge
are equal to 50%. Calculate the input capacitance and
ESR required for a specified input voltage ripple using
the following equations:
where:
and:
where:
All equations listed above are valid under the assump-
tion that the input ports of both converters can be
merged in the physical layout, so that only one input
capacitor truly serves both converters. If this is not the
case, additional low-ESR, low-ESL ceramic capacitors
should be locally placed on each converter’s input port,
connected between the drain of the high-side MOSFET
and the source of the low-side MOSFET.
Output Capacitor
The key selection parameters for the output capacitor
are capacitance value, ESR, and voltage rating. These
parameters affect the overall stability, output ripple volt-
age, and transient response. The output ripple has two
components: variations in the charge stored in the out-
put capacitor, and the voltage drop across the capaci-
tor’s ESR caused by the current flowing into and out of
the capacitor:
∆ ∆ ∆ V V V
RIPPLE ESR Q
≅ +
D
V
V
OUT
IN
=
C
I D D
V f
IN
OUT
Q SW
=
Ч
Ч
− ( ) 1
∆
∆I
V V V
V f L
L
IN OUT OUT
IN SW
=
Ч
Ч Ч
− ( )
ESR
V
I
I
IN
ESR
OUT
L
=
+
∆
∆
2
I I
V V V
V
RMS LOAD MAX
OUT IN OUT
IN
=
−
( )
( )
R
V
A
LIM
ITH
_
_ =
× 10
50 µ
V R I
LIR
ITH DS ON MAX LOAD MAX
> Ч Ч ⎛
⎝⎜
⎞
⎠⎟
−
( , ) ( )
1
2