Rainbow Electronics MAX5098A - Manual (Page 20)

MAX5098A
Dual, 2.2MHz, Automotive Buck or Boost
Converter with 80V Load-Dump Protection
20 ______________________________________________________________________________________
where
ΔI L is the peak-to-peak inductor current as calculated
above and f SW is the individual converter’s switching
frequency.
The allowable deviation of the output voltage during
fast transient loads also determines the output capaci-
tance and its ESR. The output capacitor supplies the
step load current until the controller responds with a
greater duty cycle. The response time (t
RESPONSE )
depends on the closed-loop bandwidth of the convert-
er. The high switching frequency of the MAX5098A
allows for higher closed-loop bandwidth, reducing
t
RESPONSE and the output capacitance requirement.
The resistive drop across the output capacitor ESR and
the capacitor discharge causes a voltage droop during
a step load. Use a combination of low-ESR tantalum or
polymer and ceramic capacitors for better transient
load and ripple/noise performance. Keep the maximum
output voltage deviation within the tolerable limits of the
electronics being powered. When using a ceramic
capacitor, assume 80% and 20% contribution from the
output capacitance discharge and the ESR drop,
respectively. Use the following equations to calculate
the required ESR and capacitance value:
where I STEP is the load step and t RESPONSE is the
response time of the controller. Controller response
time depends on the control-loop bandwidth.
Boost Converter
The MAX5098A can be configured for step-up conver-
sion since the internal MOSFET can be used as a low-
side switch. Use the following equations to calculate
the values for the inductor (L
MIN ), input capacitor (C IN ),
and output capacitor (C
OUT
) when using the converter
in boost operation.
Inductor
Choose the minimum inductor value so the converter
remains in continuous mode operation at minimum out-
put current (I
OMIN ).
where
The V D is the forward voltage drop of the external
Schottky diode, D is the duty cycle, and V
DS
is the volt-
age drop across the internal MOSFET switch. Select
the inductor with low DC resistance and with a satura-
tion current (I
SAT ) rating higher than the peak switch
current limit of 4.3A (I CL1 ) and 2.6A (I CL2 ) of converter
1 and converter 2, respectively.
Input Capacitor
The input current for the boost converter is continuous
and the RMS ripple current at the input is low. Calculate
the capacitor value and ESR of the input capacitor
using the following equations.
where
where V DS is the voltage drop across the internal
MOSFET switch. ΔI L is the peak-to-peak inductor ripple
current as calculated above. ΔV Q is the portion of input
ripple due to the capacitor discharge and ΔV ESR is the
contribution due to ESR of the capacitor.
Output Capacitor
For the boost converter, the output capacitor supplies
the load current when the main switch is ON. The
required output capacitance is high, especially at high-
er duty cycles. Also, the output capacitor ESR needs to
be low enough to minimize the voltage drop due to the
ESR while supporting the load current. Use the follow-
ing equation to calculate the output capacitor for a
specified output ripple tolerance.
where I PK is the peak inductor current as defined in the
Power Dissipation section for the boost converter, I O is
the load current, ΔV Q is the portion of the ripple due to
ESR
V
I
C
I D
V f
ESR
PK
OUT
O MAX
Q SW
=
=
Ч
Ч
Δ
Δ
ΔI
V V D
L f
L
IN DS
SW
=
( )
Ч
Ч
−
C
I
f V
ESR
V
I
IN
L
SW Q
ESR
L
=
Ч Ч
=
Δ
Δ
Δ
Δ
8
D
V V V
V V V
O D IN
O D DS
=
+
+
−
−
L
V D
f V I
MIN
IN
SW O OMIN
=
Ч
Ч Ч Ч
2
2
ESR
V
I
C
I t
V
OUT
ESR
STEP
OUT
STEP RESPONSE
Q
=
=
×
Δ
Δ
Δ Δ Δ V V V
O RIPPLE ESR Q _
≅ +