ISL6522_06 Datasheet, PDF (7/15 Pages) Intersil Corporation – Buck and Synchronous Rectifier Pulse-Width Modulator (PWM) Controller

ISL6522
VOLTAGE
V_{SOFT START}
V_OUT
V_OSC(MIN)
V_COMP
CLAMP ON V_COMP RELEASED AT
STEADY STATE
t₀
t₁
t₂
TIME
t₁
=
C_{-----S----S--}
I_SS
⋅
V_O
S
C
(
M
IN
)
t_SoftStart
=
t₂ – t₁
=
-^C----^S----^S--
I_SS
⋅
-^V----^O----^U----^T----^S----^t--^e---^a---^d----^y---^S----^t-^a----^t--^e-
V_IN
⋅
∆
V_O
S
C
Where:
C_SS = Soft Start Capacitor
I_SS = Soft Start Current = 10µA
V_OSC(MIN) = Bottom of Oscillator = 1.35V
V_IN = Input Voltage
∆V_OSC = Peak to Peak Oscillator Voltage = 1.9V
V_{OUTSteadyState} = Steady State Output Voltage
FIGURE 3. SOFT-START INTERVAL
4V
2V
0V
15A
10A
5A
0A
TIME (20ms/DIV)
FIGURE 4. OVERCURRENT OPERATION
Overcurrent Protection
The overcurrent function protects the converter from a
shorted output by using the upper MOSFETs on-resistance,
r_DS(ON) to monitor the current. This method enhances the
converter’s efficiency and reduces cost by eliminating a
current sensing resistor.
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OCSET)
programs the overcurrent trip level. An internal 200µA
(typical) current sink develops a voltage across R_OCSET that
7
is reference to V_IN. When the voltage across the upper
MOSFET (also referenced to V_IN) exceeds the voltage
across R_OCSET, the overcurrent function initiates a soft-start
sequence. The soft-start function discharges C_SS with a
10µA current sink and inhibits PWM operation. The soft-start
function recharges C_SS, and PWM operation resumes with
the error amplifier clamped to the SS voltage. Should an
overload occur while recharging C_SS, the soft-start function
inhibits PWM operation while fully charging C_SS to 4V to
complete its cycle. Figure 4 shows this operation with an
overload condition. Note that the inductor current increases
to over 15A during the C_SS charging interval and causes an
overcurrent trip. The converter dissipates very little power
with this method. The measured input power for the
conditions of Figure 4 is 2.5W.
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by:
I_PEAK
=
I_{--O-----C----S----E----T-----}•_----R_{----O-----C----S----E----T--}
r_DS(ON)
where I_OCSET is the internal OCSET current source (200µA
is typical). The OC trip point varies mainly due to the
MOSFETs r_DS(ON) variations. To avoid overcurrent tripping
in the normal operating load range, find the R_OCSET resistor
from the equation above with:
The maximum r_DS(ON) at the highest junction temperature.
1. The minimum I_OCSET from the specification table.
2. Determine I_PEAK for I_PEAK > I_OUT(MAX) + (∆I) ⁄ 2 ,
where ∆I is the output inductor ripple current.
For an equation for the ripple current see the section under
component guidelines titled Output Inductor Selection.
A small ceramic capacitor should be placed in parallel with
R_OCSET to smooth the voltage across R_OCSET in the
presence of switching noise on the input voltage.
Current Sinking
The ISL6522 incorporates a MOSFET shoot-through
protection method which allows a converter to sink current
as well as source current. Care should be exercised when
designing a converter with the ISL6522 when it is known that
the converter may sink current.
When the converter is sinking current, it is behaving as a boost
converter that is regulating its input voltage. This means that
the converter is boosting current into the V_IN rail, the voltage
that is being down-converted. If there is nowhere for this current
to go, such as to other distributed loads on the V_IN rail, through
a voltage limiting protection device, or other methods, the
capacitance on the V_IN bus will absorb the current. This
situation will cause the voltage level of the V_IN rail to increase. If
the voltage level of the rail is boosted to a level that exceeds the
maximum voltage rating of the MOSFETs or the input
capacitors, damage may occur to these parts. If the bias
voltage for the ISL6522 comes from the V_IN rail, then the
FN9030.8
March 10, 2006