ISL70001SRH_11 Datasheet, PDF (12/16 Pages) Intersil Corporation – Radiation Hardened and SEE Hardened 6A Synchronous Buck Regulator

ISL70001SRH
at the phase node. The filter must also provide the transient
energy until the regulator can respond. Since the filter has low
bandwidth relative to the switching frequency, it limits the
system transient response. The output capacitors must supply or
sink current while the current in the output inductor increases or
decreases to meet the load demand.
OUTPUT CAPACITOR SELECTION
The critical load parameters in choosing the output capacitors are
the maximum size of the load step (ΔI_STEP), the load-current slew
rate (di/dt), and the maximum allowable output voltage deviation
under transient loading (ΔV_MAX). Capacitors are characterized
according to their capacitance, ESR (Equivalent Series Resistance)
and ESL (Equivalent Series Inductance).
At the beginning of a load transient, the output capacitors supply all
of the transient current. The output voltage initially deviates by an
amount approximated by the voltage drop across the ESL. As the
load current increases, the voltage drop across the ESR increases
linearly until the load current reaches its final value. Neglecting the
contribution of inductor current and regulator response, the output
voltage initially deviates by an amount shown in Equation 8.
ΔV_MAX ≈
E
S
L
×
-^d----ⁱ
dt
+ [ESR × ΔI_STEP]
(EQ. 8)
The filter capacitors selected must have sufficiently low ESL and
ESR such that the total output voltage deviation is less than the
maximum allowable ripple.
Most capacitor solutions rely on a mixture of high frequency
capacitors with relatively low capacitance in combination with
bulk capacitors having high capacitance but larger ESR.
Minimizing the ESL of the high-frequency capacitors allows them
to support the output voltage as the current increases.
Minimizing the ESR of the bulk capacitors allows them to supply
the increased current with less output voltage deviation.
Ceramic capacitors with X7R dielectric are recommended.
Alternately, a combination of low ESR solid tantalum capacitors
and ceramic capacitors with X7R dielectric may be used.
The ESR of the bulk capacitors is responsible for most of the
output voltage ripple. As the bulk capacitors sink and source the
inductor AC ripple current, a voltage, V_P-P(MAX), develops across
the bulk capacitor according to Equation 9.
V_P-P(MAX) = ESR ×
(_---V_{---I--N-----–-----}V_{---O----U---T---})_---V_{---O----U----T-}
L_OUT × f_s × V_IN
(EQ. 9)
Another consideration in selecting the output capacitors is loop
stability. The total output capacitance sets the dominant pole of
the PWM. Because the ISL70001SRH uses integrated
compensation techniques, it is necessary to restrict the output
capacitance in order to optimize loop stability. The
recommended load capacitance can be estimated using
Equation 10.
C_OUT
=
75μF × NumberofLXxPinsConnected × -¹----^.-⁸----^V--
V_OUT
(EQ. 10)
OUTPUT INDUCTOR SELECTION
Once the output capacitors are selected, the maximum allowable
ripple voltage, V_P-P(MAX), determines the lower limit on the
inductance as shown in Equation 11.
L_OUT ≥ ESR ×
--⁽---^V---^I--^N-----–----^V----^O----^U---^T---⁾---^V---^O----^U---^T---
f_s × V_IN × V_P-P(MAX)
(EQ. 11)
Since the output capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the transient,
the capacitor voltage becomes slightly depleted. The output
inductor must be capable of assuming the entire load current
before the output voltage decreases more than ΔV_MAX. This
places an upper limit on inductance.
Equation 12 gives the upper limit on output inductance for the
case when the trailing edge of the current transient causes a
greater output voltage deviation than the leading edge.
Equation 13 addresses the leading edge. Normally, the trailing
edge dictates the inductance selection because duty cycles are
usually <50%. Nevertheless, both inequalities should be
evaluated, and inductance should be governed based on the
lower of the two results. In each equation, L_OUT is the output
inductance, C_OUT is the total output capacitance, and ΔI_L(P-P) is
the peak-to-peak ripple current in the output inductor.
L_OU
T
≤
2-----⋅_---C_{----O----U---T----}⋅_{---V-----O---U----T-}
(ΔI_STEP)²
ΔV_MAX – (ΔI_L(P-P) ⋅ ESR)
(EQ. 12)
L_OU
T
≤
---²-----^â‹---^C----^O----^U---^T----
(ΔI_STEP)²
ΔV_MAX – (ΔI_L(P-P) ⋅ ESR)
⎛
⎝
V_IN
–
V_OU
⎞
T_⎠
(EQ. 13)
The other concern when selecting an output inductor is to ensure
there is adequate slope compensation when the regulator is
operated above 50% duty cycle. Since the internal slope
compensation is fixed, output inductance should satisfy
Equation 14 to ensure this requirement is met.
L_OU
T
≥
--------------------------------⁴----^.-³----²----^μ----^H----------------------------------
NumberofLXxPinsConnected
(EQ. 14)
Input Capacitor Selection
Input capacitors are responsible for sourcing the AC component
of the input current flowing into the switching power devices.
Their RMS current capacity must be sufficient to handle the AC
component of the current drawn by the switching power devices,
which is related to duty cycle. The maximum RMS current
required by the regulator is closely approximated by Equation 15.
IRMSMAX =
V_{-----O-----U-----T--}
VIN
×
⎛
⎜
⎝
IOU
TM
A
2
X
+
--¹----
12
×
⎛
⎜
⎝
V_{-----I--N------}–_----V_{----O------U----T--}
L_OUT × fs
×
V_{----V-O---I--UN-----T--⎠⎟⎞}
2_⎞
⎟
⎠
(EQ. 15)
The important parameters to consider when selecting an input
capacitor are the voltage rating and the RMS ripple current
rating. For reliable operation, select capacitors with voltage
ratings at least 1.5x greater than the maximum input voltage.
The capacitor RMS ripple current rating should be higher than
the largest RMS ripple current required by the circuit.
12
FN6947.1
May 23, 2011