Lo w -Vo lt a g e , P re c is io n S t e p -Do w n
Co n t ro lle r fo r P o rt a b le CP U P o w e r
P WM Co n t ro lle r
The heart of the current-mode PWM controller is a
multi-input, open-loop comparator that sums four sig-
nals: the output voltage error signal with respect to the
reference voltage, the current-sense signal, the inte-
g ra te d volta g e -fe e d b a c k s ig na l, a nd the s lop e -
compensation ramp (Figure 3).
SKIP
Table 3.
PWM Table
LOAD
CURRENT
SKIP
MODE
DESCRIPTION
Pulse-skipping,
Low
Light
Idle
discontinuous inductor
current
The PWM controller is a direct-summing type, lacking a
traditional error amplifier and the phase shift associat-
ed with it. This direct-summing configuration approach-
es ideal cycle-by-cycle control over the output voltage
(Figure 4).
Constant-frequency PWM,
continuous inductor current
MAX136
Low
High
High
Heavy
Light
PWM
PWM
PWM
Constant-frequency PWM,
continuous inductor current
When SKIP = low, Idle Mode circuitry automatically
optimizes efficiency throughout the load-current range.
Idle Mode dramatically improves light-load efficiency
Constant-frequency PWM,
continuous inductor current
Heavy
CSH
1X
CSL
FB
2X
REF
CC
gm
BST
R
Q
LEVEL
SHIFT
DH
LX
S
OSC
SLOPE
COMPENSATION
30mV
SKIP
CURRENT
LIMIT
DAC
SHOOT-
THROUGH
CONTROL
CK
COUNTER
SHDN
SOFT-START
SYNCHRONOUS
RECTIFIER CONTROL
VL
DL
R
S
Q
LEVEL
SHIFT
-100mV
PGND
Figure 3. PWM Controller Functional Diagram
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
