ISL62870 Datasheet, PDF (7/16 Pages) Intersil Corporation – PWM DC/DC Voltage Regulator Controller

ISL62870
V_OUT R_FB
FB
SREF
−
EA
+
V_COMP
+
V_SET
−
V_REF
FIGURE 4. ISL62870 VOLTAGE PROGRAMMING CIRCUIT
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
10µ
OCSET
+ ^{_ V}ROCSET
V_O
C_O
R_O
VO
FIGURE 5. OVERCURRENT PROGRAMMING CIRCUIT
Figure 5 shows the overcurrent set circuit. The inductor
consists of inductance L and the DC resistance DCR. The
inductor DC current I_L creates a voltage drop across DCR,
which is given by Equation 5:
V_DCR = I_L ⋅ DCR
(EQ. 5)
The I_OCSET current source sinks 10µA into the OCSET pin,
creating a DC voltage drop across the resistor R_OCSET,
which is given by Equation 6:
V_ROCSET = 10μA ⋅ R_OCSET
(EQ. 6)
The DC voltage difference between the OCSET pin and the
VO pin, which is given by Equation 7:
V_OCSET– V_VO = V_DCR– V_ROCSET =I_L ⋅DCR – I_OCSET⋅ R_OCSET
(EQ. 7)
The IC monitors the voltage of the OCSET pin and the VO
pin. When the voltage of the OCSET pin is higher than the
voltage of the VO pin for more than 10µs, an OCP fault
latches the converter off.
Component Selection For R_OCSET and C_SEN
The value of R_OCSET is calculated with Equation 8 which is
written as follows:
R_OCSET
=
I_{--O-----C-----}⋅_---D_-----C_-----R_---
I_OCSET
(EQ. 8)
Where:
- R_OCSET (Ω) is the resistor used to program the
overcurrent setpoint
- I_OC is the output DC load current that will activate the
OCP fault detection circuit
- DCR is the inductor DC resistance
7
FN6708.0
August 14, 2008