Rainbow Electronics MAX1844 - Manual (Page 11)

battery supply, the enable signal (SHDN) must be
delayed until the battery voltage is present in order to
ensure startup. The 5V bias supply provides V
CC and
gate-drive power, so the maximum current drawn is:
I BIAS = I CC + f (Q G1 + Q G2 ) = 5mA to 30mA (typ)
where I CC is 550µA (typ), f is the switching frequency,
and Q G1 and Q G2 are the MOSFET data sheet total
gate-charge specification limits at V GS = 5V.
Free-Running, Constant-On-Time PWM
Controller with Input Feed-Forward
The Quick-PWM control architecture is a pseudo-fixed-fre-
quency, constant-on-time on-demand PWM with voltage
feed-forward (Figure 2). This architecture relies on the out-
put filter capacitor’s ESR to act as a current-sense resistor,
so the output ripple voltage provides the PWM ramp sig-
nal. The control algorithm is simple: the high-side switch
on-time is determined solely by a one-shot whose pulse
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
______________________________________________________________________________________ 11
Figure 2. MAX1844 Functional Diagram
REF
-10%
FROM
OUT
REF
FB
ERROR
AMP
TOFF
TON
REF
+10%
FEEDBACK
MUX
(SEE FIGURE 6)
CHIP
SUPPLY
x2
1.0V
R
0.1V
POR
OVP
9R
ILIM
V CC - 1V
V CC - 1V
V CC - 1V
SHDN
PGOOD
ON-TIME
COMPUTE
TON
1-SHOT
1-SHOT
TRIG
IN
2V TO 28V
TRIG
Q
Q
S
R
2V
REF
REF
5V
OUTPUT
DL
CS
GND
V CC
V DD
LX
ZERO CROSSING
CURRENT
LIMIT
DH
BST
5V
+5V
Q
SKIP
TON
LATCH
V+
Σ
MAX1844
S
R
Q
0.7V
1.14V
UVP
20ms
TIMER
OVP/UVP
LATCH
0.1V
OUT