isl6236 Intersil Corporation, isl6236 Datasheet - Page 25

• Manufacturer Part Number: isl6236
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

Automatic Pulse-Skipping Switchover
(Idle Mode)
In Idle Mode (SKIP# = GND), an inherent automatic
switchover to PFM takes place at light loads. This switchover
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 nonskipping 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 (t
PFM/PWM crossover occurs, I
the peak-to-peak ripple current, which is a function of the
inductor value (Figure 71). For example, in the ISL6236
typical application circuit with V
L = 7.6µH, and K = 5µs, switchover to pulse-skipping
operation occurs at I
I
LOAD SKIP
(
ON
)” on page 20). The load-current level at which
)
=
K V
----------------------- -
⋅
2 L
⋅
OUT
LOAD
PHASE
OUT
FB
VIN
VIN
ILIM
V
------------------------------- -
IN
= 0.96A or about on-fifth full load.
–
V
VCC
25
IN
V
LOAD(SKIP)
OUT
OUT1
t
ON
5µA
DECODER
FB
= 5V, V
+
+
FIGURE 70. PWM CONTROLLER (ONE SIDE ONLY)
OUT
REFIN2 (SMPS2)
VREF
, is equal to half
SLOPE COMP
IN
+
+ +
= 12V,
0.9V
1.1V
0.7V
(EQ. 3)
REF
REF
REF
+
+
+
+
+
+
+
+
+
COMP
Â
S
ISL6236
+
+ +
+
+ +
+
+
+
+
+ +
OV LATCH
UV LATCH
PGOOD
SKIP#
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
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
20ms
BLANKING
0
Q
ONE SHOT
S
S
S
R
R
R
MIN. t
Δ I
FIGURE 71. ULTRASONIC CURRENT WAVEFORMS
t
t
t
t
DETECT
Q
Q
BOOT
Q
Q
Q
ON-TIME
UV
=
=
=
=
LATCH
FAULT
LOGIC
R
R
R
S
S
S
LATCH
OFF
FAULT
TRIG
Q
Q
Q
Q
Q
V IN -V OUT
V IN -V
V IN -V
V
-V
L
L
L
TO UGATE DRIVER
ONE-SHOT
TO LGATE DRIVER
SMSP1 ONLY
+
BOOT
SECFB
TIME
2V
I
LOAD
I PEAK
I PEAK
I PEAK
I PEAK
June 28, 2007
= I
PEAK/2
FN6373.3