ISL78206 Datasheet, PDF (15/19 Pages) Intersil Corporation – 40V 2.5A Buck Controller with Integrated High-side 40V 2.5A Buck Controller with Integrated High-side

ISL78206
Regarding transient response needs, a good starting point is to
determine the allowable overshoot in V_OUT if the load is suddenly
removed. In this case, energy stored in the inductor will be
transferred to C_OUT causing its voltage to rise. After calculating
capacitance required for both ripple and transient needs, choose
the larger of the calculated values. The following equation
determines the required output capacitor value in order to
achieve a desired overshoot relative to the regulated voltage.
C_OUT = _V-----_O----_U----_T----₂----_*---_--_V----_O-----_U^I--^O--_T--^U--_M--^T--_A--²--_X--^*---^L_---_V----_O-----_U----_T---_---₂-----–----₁-----_
(EQ. 6)
Where V_OUTMAX/V_OUT is the relative maximum overshoot
allowed during the removal of the load.
Input Capacitors
Depending upon the system input power rail conditions, the
aluminum electrolytic type capacitor is normally needed to
provide the stable input voltage and restrict the switching
frequency pulse current in small areas over the input traces for
better EMC performance. The input capacitor should be able to
handle the RMS current from the switching power devices.
Ceramic capacitors must be used at the VIN pin of the IC and
multiple capacitors, including 1µF and 0.1µF, are recommended.
Place these capacitors as closely as possible to the IC.
Output Inductor
The inductor value determines the converter’s ripple current.
Choosing an inductor current requires a somewhat arbitrary
choice of ripple current, I. A reasonable starting point is 30% to
40% of total load current. The inductor value can then be
calculated using Equation 7:
L = -^V----^I--^N_F----_s-–----_^V----_-^O---_I-^U----^T--  ^V----_V-^O---_I-^U-_N---^T--
(EQ. 7)
Increasing the value of inductance reduces the ripple current and
thus ripple voltage. However, the larger inductance value may
reduce the converter’s response time to a load transient. The
inductor current rating should be such that it will not saturate in
overcurrent conditions.
Low Side Power MOSFET
In synchronous buck application, a power N MOSFET is needed
as the synchronous low side MOSFET and a good one should
have low Qgd, low r_DS(ON) and small Rg (Rg_typ <1.5Ω
recommended). Vgth_min is recommended to be or higher than
1.2V. A good example is SQS462EN.
In synchronous buck configuration, a 5.1k or smaller value
resistor has to be added to connect LGATE to ground to avoid
falsely turn-on of LGATE caused by coupling noise.
Output Voltage Feedback Resistor Divider
The output voltage can be programmed down to 0.8V by a
resistor divider from V_OUT to FB, according to Equation 8.
V_OUT
=
0.8




1
+
R^---R--_L^---UO----P-_W^---_{}
(EQ. 8)
In applications requiring the least input quiescent current, large
resistors should be used for the divider to keep its leakage
current low. Generally, a resistor value of 10k to 300k can be
used for the upper resistor.
Loop Compensation Design
The ISL78206 uses constant frequency peak current mode
control architecture to achieve fast loop transient response. An
accurate current sensing pilot device in parallel with the upper
MOSFET is used for peak current control signal and overcurrent
protection. The inductor is not considered as a state variable
since its peak current is constant, and the system becomes
single order system. It is much easier to design the compensator
to stabilize the loop compared with voltage mode control. Peak
current mode control has inherent input voltage feed-forward
function to achieve good line regulation. Figure 22 shows the
small signal model of a buck regulator.
^iin
V^in
+
^iL _LP
R_LP
ILd^ ^{1:D Vin d^}
RT
vo^
Rc
Ro
Co
d^
Fm
T _i(S)
+
He(S)
Tv(S)
v^comp
-Av(S)
FIGURE 22. SMALL SIGNAL MODEL OF BUCK REGULATOR
PWM Comparator Gain F_m
The PWM comparator gain F_m for peak current mode control is
given by Equation 9:
F_m = _v-ˆ---_c---_o^d-ˆ--_m-----_p- = -_--_S----_e-----+---¹-_S-----_n---_---_T---_s-
(EQ. 9)
Where S_e is the slew rate of the slope compensation and S_n is
given by Equation 10.
S_n = R_t^V-----ⁱ-ⁿ--_L---–_P----^V-----^o-
(EQ. 10)
Where R_t is the gain of the current amplifier.
Current Sampling Transfer Function H_e(S)
In current loop, the current signal is sampled every switching
cycle. It has the following transfer function in Equation 11:
H_eS= _-^S----²ⁿ²- + _-----_n-^S--_Q-----_n- + 1
Where Q_n and _n are given by Q_n = –²-_- _n= f_s
(EQ. 11)
Submit Document Feedback 15
FN8618.2
March 25, 2015