ISL62870HRUZ-T Intersil, ISL62870HRUZ-T Datasheet - Page 7

• Manufacturer Part Number: ISL62870HRUZ-T
• Description: IC CTRLR VREG PWM DC/DC 16-TQFN
• Manufacturer: Intersil

Specifications of ISL62870HRUZ-T

• Pwm Type: Controller
• Number Of Outputs: 1
• Frequency - Max: 330kHz
• Duty Cycle: 100%
• Voltage - Supply: 3.3 V ~ 25 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -10°C ~ 100°C
• Package / Case: 16-UTQFN (16-µTQFN)
• Frequency-max: 330kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Soft-Start Delay
Circuit Description
When the voltage on the VCC pin has ramped above the
rising power-on reset voltage
the EN pin has increased above the rising enable threshold
voltage
and enables the reference amplifier V
current I
pin and charges capacitor C
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
delay t
Where:
The end of soft-start is detected by I
capacitor C
set, the PGOOD pin goes high, and diode emulation mode
(DEM) is enabled.
Component Selection For C
Choosing the C
particular soft-start delay t
which is written as follows:
Where:
C
t
SS
SREF
SOFT
- I
- V
- t
- I
- V
FIGURE 4. ISL62870 VOLTAGE PROGRAMMING CIRCUIT
V
OUT
=
limit
SS
SS
SS
SREF
SREF
SS
V
------------------------------------------ -
has charged C
V
=
SREF
SS
is the soft-start current source at the 20µA limit
is the soft-start delay
is the 20µA soft-start current source at the 20µA
ENTHR
R
which is given by Equation 3:
t
---------------------- -
SS
V
FB
SOFT
is limited to 20µA and is sourced out of the SREF
is the buffered V
SREF
is the buffered V
I
SS
⋅
⋅
C
I
SOFT
SS
, the SREF pin releases its discharge clamp,
SOFT
charges to V
SREF
capacitor to meet the requirements of a
FB
SOFT
SS
SOFT
to V
REF
REF
is calculated using Equation 4,
V
REF
7
VCC_THR
REF
SOFT
reference voltage
reference voltage
. The internal SSOK flag is
until V
SS
−
+
is called the soft-start
SET
EA
tapering off when
V
, and the voltage on
Capacitor
SREF
SET
. The soft-start
+
−
V
V
equals V
COMP
REF
(EQ. 3)
(EQ. 4)
REF
ISL62870
.
Fault Protection
Overcurrent
The overcurrent protection (OCP) setpoint is programmed
with resistor R
OCSET and PHASE pins. Resistor R
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
of resistor R
Figure 5 shows the overcurrent set circuit. The inductor
consists of inductance L and the DC resistance DCR. The
inductor DC current I
which is given by Equation 5:
The I
creating a DC voltage drop across the resistor R
which is given by Equation 6:
The DC voltage difference between the OCSET pin and the
VO pin, which is given by Equation 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
The value of R
written as follows:
Where:
V
V
R
V
O
DCR
ROCSET
OCSET
OCSET
- R
- I
- DCR is the inductor DC resistance
. The value of resistor R
10µ
FIGURE 5. OVERCURRENT PROGRAMMING CIRCUIT
overcurrent setpoint
OCP fault detection circuit
OCSET
OC
OCSET
=
PHASE
OCSET
–
is the output DC load current that will activate the
I
L
=
V
=
⋅
I
--------------------------- -
VO
VO
DCR
OCSET
OC
current source sinks 10µA into the OCSET pin,
10μA R
I
OCSET
(Ω) is the resistor used to program the
OCSET
=
OCSET
⋅
V
DCR
DCR
⋅
.
+
+
L
OCSET
, which is connected across the
V
–
R
is calculated with Equation 8 which is
creates a voltage drop across DCR,
ROCSET
V
OCSET
DCR
ROCSET
R
O
O
should always match the value
_
V
DCR
C
=
OCSET
SEN
I
L
L
⋅
O
DCR
is connected
and C
I
–
L
_
I
OCSET
OCSET,
SEN
August 14, 2008
V
⋅
O
R
FN6708.0
(EQ. 5)
(EQ. 6)
OCSET
(EQ. 8)
(EQ. 7)
C
O