Rainbow Electronics MAX15046B - Manual (Page 15)

40V, High-Performance, Synchronous
Buck Controller
MAX15046
______________________________________________________________________________________ 15
Connect an external resistor (R LIM ) from LIM to GND
to adjust the current-limit threshold, which is temper-
ature-compensated with a temperature coefficient of
-2300ppm/NC. The relationship between the current-limit
threshold (V
ITH ) and R LIM is:
ITH
LIM
6
MAX AMB)
10 V
R
50 10 1 2300 (T - T
−
Ч
=
  Ч Ч + Ч
 
where R LIM is in I, V ITH is in V, T MAX and T AMB are in
N C.
An R LIM resistance range of 6kI to 60kI corresponds
to a current-limit threshold of 30mV to 300mV. Use 1%
tolerance resistors when adjusting the current limit to
minimize error in the current-limit threshold.
Input Capacitor
The input filter capacitor reduces peak current drawn
from the power source and reduces noise and voltage
ripple on the input caused by the switching circuitry. The
input capacitor must meet the ripple current requirement
(I
RMS ) imposed by the switching currents as defined by
the following equation:
OUT IN OUT
RMS LOAD(MAX)
IN
V (V - V )
I I
V
=
I RMS attains a maximum value when the input volt-
age equals twice the output voltage (V IN = 2V OUT ),
so I RMS(MAX) = I LOAD(MAX)/2 . For most applications,
nontantalum capacitors (ceramic, aluminum, polymer, or
OS-CON) are preferred at the inputs due to the robust-
ness of nontantalum capacitors to accommodate high
inrush currents of systems being powered from very low
impedance sources. Additionally, two (or more) smaller-
value low-ESR capacitors should be connected in paral-
lel to reduce high-frequency noise.
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 output
capacitor, and the voltage drop across the capacitor’s
ESR caused by the current flowing into and out of the
capacitor:
D
V RIPPLE = DV ESR + DV Q
The output-voltage ripple as a consequence of the ESR
and the output capacitance is:
ESR P-P
P-P
Q
OUT SW
IN OUT OUT
P-P
SW IN
V I ESR
I
V
8 C f
V - V V
I
f L V
∆ = Ч
∆ =
Ч Ч
   
= Ч    
Ч
   
where I P-P is the peak-to-peak inductor current ripple
(see the Inductor Selection section). Use these equa-
tions for initial capacitor selection. Decide on the final
values by testing a prototype or an evaluation circuit.
Check the output capacitor against load-transient
response requirements. The allowable deviation of the
output voltage during fast load transients determines
the capacitor output capacitance, ESR, and equivalent
series inductance (ESL). The output capacitor supplies
the load current during a load step until the controller
responds with a higher duty cycle. The response time
(t
RESPONSE ) depends on the closed-loop bandwidth of
the converter (see the Compensation Design section).
The resistive drop across the ESR of the output capaci-
tor, the voltage drop across the ESL (DV
ESL ) of the
capacitor, and the capacitor discharge, cause a voltage
droop during the load step.
Use a combination of low-ESR tantalum/aluminum elec-
trolytic and ceramic capacitors for improved transient
load and voltage ripple performance. Nonleaded capac-
itors and capacitors in parallel help reduce the ESL.
Keep the maximum output-voltage deviation below the
tolerable limits of the load. Use the following equations to
calculate the required ESR, ESL, and capacitance value
during a load step:
∆
=
Ч
=
∆
∆ Ч
=
≅
Ч
ESR
STEP
STEP RESPONSE
OUT
Q
ESL STEP
STEP
RESPONSE
O
V
ESR
I
I t
C
V
V t
ESL
I
1
t
3 f
where I STEP is the load step, t STEP is the rise time of the
load step, t RESPONSE is the response time of the control-
ler, and f O is the closed-loop crossover frequency.