ISL6236 Datasheet, PDF (25/35 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236
t_ON
VIN
ILIM
5µA
+
OUT
REFIN2 (SMPS2)
VREF
SLOPE COMP ^COMP
MIN. t_OFF
Q
TRIG
ONE SHOT
R QQ
S QQ
TO UGATE DRIVER
BOOT
UV
DETECT
BOOT
VCC
PHASE
OUT
ÂS
SKIP
S QQ
R _QQ
TO LGATE DRIVER
ONE-SHOT
SECFB
FB
DECODER
FB
0.9V_REF
1.1V_REF
0.7V_REF
PGOOD
OV LATCH
UV LATCH
FAULT
LLFAAATTUCCLHTH
LOGIC
20ms
BLANKING
2V
SMSP1 ONLY
FIGURE 70. PWM CONTROLLER (ONE SIDE ONLY)
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 non-skipping
PWM operation to coincide with the boundary between
continuous and discontinuous inductor-current operation (also
known as the critical conduction point):
I_LOAD(SKIP)
=
-^K-----^â‹-₂--^V---_â‹-^O--_L---^U----^T--
-^V----^I--^N-----–----^V-----^O----^U----^T--
V_IN
(EQ. 3)
where K is the ON-time scale factor (see “ON-TIME ONE-
SHOT (t_ON)” on page 20). The load-current level at which
PFM/PWM crossover occurs, I_LOAD(SKIP), is equal to half
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_OUT1 = 5V, V_IN = 12V,
L = 7.6µH, and K = 5µs, switchover to pulse-skipping
operation occurs at I_LOAD = 0.96A or about on-fifth full load.
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
ΔI
=
t
VIN -VOUT
L
IPEAK
I_LOAD = I_PEAK/2
0 ON-TIME
TIME
FIGURE 71. ULTRASONIC CURRENT WAVEFORMS
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
25
FN6373.6
April 29, 2010