ISL78210 Datasheet, PDF (7/17 Pages) Intersil Corporation – Automotive PWM DC/DC Voltage Controller

ISL78210
Setpoint Reference Voltage
The 500mV output of the setpoint reference voltage
(V_SREF) appears at the SREF pin. This signal is the
output of the current limited voltage follower that buffers
an internal 500mV voltage reference (V_REF.) The
converter is in regulation when the voltage at the FB pin
(V_FB) equals the V_SREF voltage at the SREF pin. Both of
these pins are measured relative to the GND pin, not the
PGND pin.
The feedback voltage-divider network consisting of offset
resistor (R_OFS) and loop-compensation resistor (R_FB)
scale down the converter output voltage (V_OUT) such
that the voltage V_FB equals V_SREF when V_OUT equals the
desired output voltage of the converter. The
voltage-divider relation is given in Equation 1:
V_FB
=
V_O
U
T
⋅
----------^R----^O-----^F---^S-----------
R_FB + R_OFS
(EQ. 1)
Where:
- V_FB = V_SREF
- R_FB is the loop-compensation feedback resistor
that connects from the FB pin to the converter
output
- R_OFS is the voltage-scaling programming resistor
that connects from the FB pin to the GND pin
The value of offset resistor R_OFS must be recalculated
whenever the value of loop-compensation resistor R_FB
has been changed. Calculation of R_OFS is written as
shown in Equation 2:
R_OFS
=
---^V-----^S----^R----^E----^F----^â‹---^R-----^F----^B----
V_OUT – V_SREF
(EQ. 2)
V_OUT R_FB
FB
−
EA
+
V_COMP
+
V_SET
−
V_REF
SREF
FIGURE 4. ISL78210 VOLTAGE PROGRAMMING
Soft-Start Delay
Circuit Description
When the voltage on the VCC pin has ramped above the
rising power-on reset voltage V_{VCC_THR}, and the voltage
on the EN pin has increased above the rising enable
threshold voltage V_ENTHR, the SREF pin releases its
discharge clamp, and enables the reference amplifier
V_SET. The soft-start current I_SS is limited to 20µA and is
sourced out of the SREF pin and charges capacitor C_SOFT
until V_SREF equals V_REF. The regulator controls the PWM
such that the voltage on the FB pin tracks the rising
voltage on the SREF pin. The elapsed time from when the
EN pin is asserted to when V_SREF has charged C_SOFT to
V_REF is called the soft-start delay t_SS which is given by
Equation 3:
t_SS
=
V_{-----S----R----E----F----}⋅_---C_{-----S----O----F----T--}
I_SS
(EQ. 3)
Where:
- I_SS is the soft-start current source at the 20µA
limit
- V_SREF is the buffered V_REF reference voltage
The end of soft-start is detected by I_SS tapering off when
capacitor C_SOFT charges to V_REF. The internal SSOK flag
is set, the PGOOD pin goes high, and diode emulation
mode (DEM) is enabled.
Component Selection For C_SOFT Capacitor
Choosing the C_SOFT capacitor to meet the requirements
of a particular soft-start delay t_SS is calculated using
Equation 4, which is written as follows:
C_SOFT
=
t_{--S----S-----}⋅_---I_--S----S--
V_SREF
(EQ. 4)
Where:
- t_SS is the soft-start delay
- I_SS is the 20µA soft-start current source at the
20µA limit
- V_SREF is the buffered V_REF reference voltage
Fault Protection
Overcurrent
The overcurrent protection (OCP) setpoint is
programmed with resistor R_OCSET, which is connected
across the OCSET and PHASE pins. Resistor R_O is
connected between the VO pin and the actual output
voltage of the converter. During normal operation, the
VO pin is a high impedance path, therefore there is no
voltage drop across R_O. The value of resistor R_O should
always match the value of resistor R_OCSET.
PHASE
L
DCR
I_L
+
V_DCR
_
R_OCSET
C_SEN
10µA
OCSET
+ ^{_ V}ROCSET
V_O
C_O
R_O
VO
FIGURE 5. OVERCURRENT PROGRAMMING CIRCUIT
7
FN7583.0
March 8, 2010