ISL78268 Datasheet, PDF (31/33 Pages) Intersil Corporation – Integrated 2A sourcing

ISL78268
The transfer function at the feedback resistor network is:
H_e1S
=
-_R----_F---_B---^R₀----^F-+--^B---_R⁰----_F---_B---₁--

-¹-----+------^-------^s-z-----^1----
1
+
----^s------
_p1
(EQ. 34)
Where,
_z1 = -_C---₁-----_---_--_R----_F---¹_B----₁-----+----_R-----₁---_
_p1 = -_C---₁-----_---_R---------F------_B-------_{1---------}------_R--------_F------_B--------₀---_R------+---F------_BR----¹---_{1--F------B-+------1}----_R-----_---F---_R---B------₁-_{0-------+----------R}--------_F-------_B-------0---------_{------R--------1---}
The total transfer function with compensation network and gain
stage will be expressed;
G_opens = G_Ts  H_e1s  H_e2s
(EQ. 35)
G_opens
=





-^R----^o-
R_s


1_{---1---+--+---}-----_{------e-}^ss_{------o---s------r}







R^-----_F^---_B^---R_O^{----F-+--B---}R^-0---_F^---_B^----₁^-

-₁₁-----++-----_--^-----------_ss^-_pz--------^1-₁-----_{}







_-----1-
s

-₁₁-----++-----_--^-----------_ss^-_pz--------^2-₂-----_{}
(EQ. 36)
From Equation 36, desired pole and zero locations can be
determined as in Equations 37 through 42.
f_po
=
---^----^o---
2
=
-----------------------¹------------------------
2    C_OUT  R_OUT
(EQ. 37)
f_z1
=
-^-----^z--¹--
2
=
2^------^---^----^---C^----₁^-----^---^1--R^----_F^---_B^----₁^----+-----R^----₁^----
f_z2
=
-^-----^z--²--
2
=
-------------------------¹---------------------------
2    C_CMc1  R_CMP
f_p1
=
-^----^p---¹--
2
=
-₂---_----_C----₁----_--_R----_F---_B----₁----_---_R----_F-^R--_B---^F-₀--^B---+¹----_R-+----_F-^R--_B--^F-₁--^B---_²---_R----₁----+-----_R----_F----_B---₀-----_---_R----₁----_
f_p2
=
-^----^p---²--
2
=
--------------------------¹---------------------------
2    C_CMP2  R_CMP
f_zesr
=
-^----^e---^s---^r
2
=
----------------------¹-----------------------
2    C_OUT  R_esr
(EQ. 38)
(EQ. 39)
(EQ. 40)
(EQ. 41)
(EQ. 42)
In general, set f_z2 and f_z1 close to the f_po. Set f_p2 near the
desired bandwidth. Set f_p1 close to f_zesr.
VCC Input Filter
To provide a quiet power rail to the internal analog circuitry, it is
recommended to place RC filter between PVCC and VCC. A 10Ω
resistor between PVCC and VCC and at least 1µF ceramic
capacitor from VCC to GND are recommended.
Current Sense Circuit
To set the current sense resistor, the voltage across the current
sense resistor should be limited to less than 0.3V. In a typical
application, it is recommended to set the voltage across the
current sense resistor between 30mV to 100mV for the typical
load current condition.
Layout Consideration
For DC/DC converter design, the PCB layout is a very important to
ensure the desired performance.
1. Place the input ceramic capacitor as close as possible to the
VIN pin and power ground connecting to the power MOSFET.
Keep this loop (input ceramic capacitor, IC VIN pin and
MOSFET) as small as possible to reduce voltage spikes
induced by the trace parasitics.
2. Place the input aluminum capacitor close to IC VIN and
ceramic capacitors.
3. Keep the phase node copper area small but large enough to
handle the load current.
4. Place the output ceramic and aluminum capacitors as close
as possible to the power stage components.
5. Place multiple vias under the thermal pad of the IC. The
thermal pad should be connected to the ground copper plane
with as large an area as possible in multiple layers to
effectively reduce the thermal impedance.
6. Place the 4.7µF decoupling ceramic capacitor at the VCC pin
and as close as possible to the IC. Put multiple vias close to
the ground pad of this capacitor.
7. Keep the bootstrap capacitor as close as possible to the IC.
8. Keep the driver traces as short as possible and try to avoid
using a via in the driver path to achieve the lowest impedance.
9. Place the current sense resistor as close as possible to the IC.
Keep the traces of current sense lines symmetric to each
other to avoid undesired switching noise injections.
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FN8657.3
December 12, 2014