ISL8112 Intersil Corporation, ISL8112 Datasheet - Page 17

• Manufacturer Part Number: ISL8112
• Description: Main Power Supply Controllers
• Manufacturer: Intersil Corporation

Automatic Pulse-Skipping Switch Over
(Idle Mode)
In Idle Mode (MODE = GND), an inherent automatic switch
over to PFM takes place at light loads. This switch over is
affected by a comparator that truncates the low-side switch
on-time at the inductor current's zero crossing. This
mechanism causes the threshold between pulse-skipping
PFM and non-skipping PWM operation to coincide with the
boundary between continuous and discontinuous
inductor-current operation (also known as the critical
conduction point):
where K is the on-time scale factor (see “On-Time One-Shot
(FS)” on page 12). The load-current level at which
PFM/PWM crossover occurs, I
the peak-to-peak ripple current, which is a function of the
inductor value (Figure 22). For example, in the ISL8112
typical application circuit with VOUT1 = 5V, V
L = 7.6µH, and K = 5µs, switch over to pulse-skipping
operation occurs at I
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
I
LOAD SKIP
(
)
=
K V
----------------------- -
⋅
2 L
⋅
OUT
LOAD
PH_
VSEN_
FB_
VIN
VIN
ILIM_
V
------------------------------- -
IN
= 0.96A or about on-fifth full load.
VCC
–
V
17
IN
V
FS
LOAD(SKIP)
OUT
5µA
DECODER
FB
+
+
OUT2REF (SMPS2)
VSEN_
FIGURE 21. PWM CONTROLLER (ONE SIDE ONLY)
VREF
SLOPE COMP
, is equal to half
IN
+
+ +
= 12V,
0.9V
1.1V
0.7V
(EQ. 3)
REF
REF
REF
+
+
+
+
+
+
+
+
+
COMP
Â
S
ISL8112
+
+ +
+
+ +
+
+
+
+
+ +
OV_LATCH_
UV_LATCH_
PGOOD_
MODE
The switching waveforms may appear noisy and
asynchronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM noise
vs. light-load efficiency are made by varying the inductor
value. Generally, low inductor values produce a broader
efficiency vs. load curve, while higher values result in higher
full-load efficiency (assuming that the coil resistance remains
fixed) and less output voltage ripple. Penalties for using
20ms
BLANKING
Q
ONE SHOT
0
S
S
S
R
R
R
Min. t
DETECT
FIGURE 22. ULTRASONIC CURRENT WAVEFORMS
Q
Q
BOOT
Q
Q
Q
ΔI
UV
t
LATCH
I
I
I
ON-TIME
FAULT
LOGIC
R
R
R
S
S
S
OFF
LATCH
FAULT
TRIG
Q
Q
Q
Q
Q
=
V IN -V OUT
V IN -V
V IN -V
V IN -V
TO UG_DRIVER
L
L
L
L
TO LG_ DRIVER
BOOT_
TIME
I
LOAD
November 21, 2006
I PEAK
I PEAK
I PEAK
I PEAK
=
I
PEAK
FN6396.0
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