ISL6540A Intersil Corporation, ISL6540A Datasheet - Page 11

• Manufacturer Part Number: ISL6540A
• Description: Single-Phase Buck PWM Controller
• Manufacturer: Intersil Corporation

Functional Description
Initialization
The ISL6540A automatically initializes upon receipt of power
without requiring any special sequencing of the input
supplies. The Power-On Reset (POR) function continually
monitors the input supply voltages (PVCC, VFF, VCC) and
the voltage at the EN pin. Assuming the EN pin is pulled to
above ~0.50V, the POR function initiates soft-start operation
after all input supplies exceed their POR thresholds.
With all input supplies above their POR thresholds, driving
the EN pin above 0.50V initiates a soft-start cycle. In addition
to normal TTL logic, the enable pin can be used as a voltage
monitor with programmable hysteresis through the use of the
internal 10μA sink current and an external resistor divider.
This feature is especially designed for applications that have
input rails greater than a 3.3V and require a specific input rail
POR and Hysteresis levels for better undervoltage
protection. Consider for a 12V application choosing
R
rising threshold (V
(V
should be taken to prevent the voltage at the EN pin from
exceeding VCC when using the programmable UVLO
functionality.
Soft-Start
The POR function activates the internal 37μA OTA which
begins charging the external capacitor (C
target voltage of VCC. The ISL6540A’s soft-start logic
UP
EN_FTH
= 100kΩ and R
PVCC POR
VCC POR
VFF POR
V
R
FIGURE 1. SOFT-START INITIALIZATION LOGIC
R
HIGH = ABOVE POR; LOW = BELOW POR
EN POR
EN_FTH
DOWN
UP
) to ~9V, for 1V of hysteresis (V
R
DOWN
=
FIGURE 2. ENABLE POR CIRCUIT
R
V
------------------------- -
I
UP
VIN
EN_HYS
=
EN_HYS
=
-------------------------------------------------------- -
V
EN_RTH
V
EN_FTH
R
EN_RTH
DOWN
UP
V
•
REF
V
) to ~10V and the falling threshold
–
= 5.76kΩ there by setting the
EN_REF
–
V
AND
I
11
EN_HYS
V
EN_REF
EN_HYS
=10
μ
SS
A
Sys_Enable
EN_HYS
) on the SS pin to a
SOFT-START
). Care
ISL6540A
continues to charge the SS pin until the voltage on COMP
exceeds the bottom of the oscillator ramp, at which point, the
driver outputs are enabled, with the low side MOSFET first
being held low for 200ns to provide for charging of the bootstrap
capacitor. Once the driver outputs are enabled, the OTA’s target
voltage is then changed to the margined (if margining is being
used) reference voltage (V
ramped up or down accordingly. This method reduces startup
surge currents due to a pre-charged output by inhibiting
regulator switching until the control loop enters its linear region.
By ramping the positive input of the error amplifier to VCC and
then to V
currents from outputs that are pre-charged above the set output
voltage. As the SS pin connects directly to the non-inverting
input of the error amplifier, noise on this pin should be kept to a
minimum through careful routing and part placement. To
prevent noise injection into the error amplifier the SS capacitor
should be located within 150 mils of the SS and GND pins.
Soft-start is declared done when the drivers have been enabled
and the SS pin is within ±3mV of V
Power Good
The power good comparator references the voltage on the
soft-start pin to prevent accidental tripping during margining.
The trip points are shown on Figure 3. Additionally, power
good will not be asserted until after the completion of the
soft-start cycle. A 0.1
an additional ~7ms delay to the assertion of power good.
PG_DLY does not delay the de-assertion of power good.
Under and Overvoltage Protection
The Undervoltage (UV) and Overvoltage (OV) protection
circuitry compares the voltage on the VMON pin with the
reference that tracks with the margining circuitry to prevent
accidental tripping. UV and OV functionality is not enabled
until the end of soft-start.
T
PG_DLY
FIGURE 3. UNDERVOLTAGE-OVERVOLTAGE WINDOW
REF_MARG
=
UV
C
PG_DLY
GOOD
, it is even possible to mitigate surge
μ
⋅
F capacitor at the PG_DLY pin will add
1.49V
--------------- -
21μA
REF_MARG
VMON
OV
REF_MARG
), and the SS pin is
GOOD
.
UV
V
REF_MARG
March 12, 2007
+15%
+9%
-9%
-15%
FN6288.2