Datasheet

ripple is mainly composed of ΔV
Q
(caused by the capaci-
tor discharge) and ΔV
ESR
(caused by the voltage drop
across the ESR of the output capacitor). Normally, a good
approximation of the output-voltage ripple is ΔV
RIPPLE
≈
ΔV
ESR
+ ΔV
Q
. If using ceramic capacitors, assume the
contribution to the output-voltage ripple from the ESR
and the capacitor discharge to be equal to 20% and 80%,
respectively. If using aluminum electrolyte capacitors,
assume the contribution to the output-voltage ripple from
the ESR and the capacitor discharge to be equal to 90%
and 10%, respectively.
Use the following equations for calculating the output
capacitance and its ESR for required peak-to-peak
output-voltage ripple.
PP
OUT
Q SW
ESR
PP
I
C
16 V f
V
ESR
I
−
−
∆
=
×∆ ×
∆
=
∆
ΔI
P-P
is the peak-to-peak inductor current and f
SW
is the
converter’s switching frequency.
The allowable deviation of the output voltage during
fast-load transients also determines the output capaci-
tance, its ESR, and its equivalent series inductance
(ESL). The output capacitor supplies the load current
during a load step until the controller responds with a
greater duty cycle. The response time (t
RESPONSE
)
depends on the closed-loop bandwidth of the converter
(see the Compensation Network section). The resis-
tive drop across the output capacitor’s ESR, the drop
across the capacitor’s ESL, and the capacitor discharge,
causes a voltage drop during the load step. Use a com-
bination of low-ESR tantalum/aluminum electrolytic and
ceramic capacitors for better transient-load and voltage-
ripple performance. Nonleaded capacitors and/or multiple
parallel capacitors help reduce the ESL. Keep the
maximum output-voltage deviation below the tolerable
limits of the electronics being powered. Use the follow-
ing equations to calculate the required ESR, ESL, and
capacitance value during a load step:
ESR
STEP
STEP RESPONSE
OUT
Q
ESL STEP
STEP
V
ESR
I
It
C
V
Vt
ESL
I
∆
=
∆
×
=
∆
∆×
=
where I
STEP
is the load step, t
STEP
is the rise time of the
load step, and t
RESPONSE
is the response time of the
controller. The response time of the converter is approxi-
mately one third of the inverse of its closed-loop band-
width and also depends on the phase margin.
Rectier Selection
The MAX5096/MAX5097 require an external Schottky/
fast-recovery diode rectifier as a freewheeling diode.
Connect this rectifier close to the device using short
leads and short PCB traces. Choose a rectifier with
a continuous current rating greater than the highest
output current-limit threshold (1.9A) and with a voltage
rating greater than the maximum expected input volt-
age, V
IN
. Use a low forward-voltage-drop Schottky recti-
fier to limit the negative voltage at LX. Avoid higher than
necessary reverse-voltage Schottky rectifiers that have
higher forward-voltage drops. Use a 60V (max) Schottky
rectifier with a 2A current rating. The Schottky rectifier
leakage current at high temperature significantly increas-
es the quiescent current in LDO mode. In applications
where LDO mode quiescent current is important, use an
ultra-fast switching diode to limit the leakage current. In
this type of application, use MURS105, MURS120 for
their fast-switching and lowleakage features.
Input Capacitor Selection
The discontinuous input current of the buck converter
causes large input-ripple currents and therefore, the input
capacitor must be carefully chosen to keep the input
voltage ripple within design requirements. The input volt-
age ripple is comprised of ΔV
Q
(caused by the capacitor
discharge) and ΔV
ESR
(caused by the ESR of the input
capacitor). The total voltage ripple is the sum of ΔV
Q
and ΔV
ESR
. Calculate the input capacitance and ESR
required for a specified ripple using the following equa-
tions (continuous mode):
ESR
PP
OUT_MAX
OUT_MAX
IN
Q SW
IN OUT OUT
PP
IN SW
OUT
IN
V
ESR
I
I
2
I D(1 D)
C
Vf
where
(V V ) V
I and
Vf L
V
D
V
−
−
∆
=
∆

+


×−
=
∆×
−×
∆=
××
=
I
OUT_MAX
is the maximum output current and D is the
duty cycle.
MAX5096/MAX5097 40V, 600mA Buck Converters with Low-
Quiescent-Current Linear Regulator Mode
www.maximintegrated.com
Maxim Integrated │ 15