Rainbow Electronics MAX1800 - Manual (Page 21)

Choose the MOSFET for low dropout. The maximum
acceptable on-resistance of the MOSFET is determined
by the maximum load current to achieve the required
dropout voltage (minimum input voltage minus output
voltage):
Determine the minimum output capacitance as follows.
The output capacitor and load resistance set the domi-
nant pole (f
P1 ):
The second pole (f P2 ) occurs due to the AO output
resistance and the gate capacitance of the external
MOSFET:
where R AO is the output resistance of the gain block at
AO, and C (GATE-Q1) is the total gate capacitance of the
MOSFET, Q1. The control loop DC gain is:
where A V-GB is the voltage gain from AI to AO (typically
100), and G
(FS-Q1)
is the forward transconductance
gain of Q1. Choose the output capacitance so that the
second pole occurs at or above the loop-gain crossover
frequency:
Since V REF is 1.25V, A (V-GB) is typically 100, and R AO
is typically 800 Ω, then:
Using the Linear Regulator to Make a
Step-Up/Step-Down Circuit
Some applications have a battery voltage that can be
either greater than or less than the desired output volt-
age. In this case, a step-up or step-down converter will
not be able to generate the required output voltage
under all conditions. To avoid this limitation, use a step-
C
G C
V
OUT
FS Q GATE Q
OUT
≥






−
( )
−
( ) , 12 500
1 1
C
V
V
A G R C
OUT
REF
OUT
V GB FS Q AO GATE Q
≥

 

−
( )
−
( )
−
( )
1 1
A
V
V
A G R
VLOOP
REF
OUT
V GB FS Q LOAD
=

 

−
( )
−
( )
1
f
R C
P
AO GATE Q
2
1
1
2
( )
=
−
π
f
R C
P
LOAD OUT
1
1
2
=
π
R
DS ON −
≤
V
I
DROPOUT
LOAD(MAX)
R R
V
V
OUT
REF
1 2 1 = −

 

MAX1800
Digital Camera Step-Up
Power Supply
______________________________________________________________________________________ 21
Figure 11. Linear Regulator
C
OUT
Q 1
V REF
R 1
R 2
LOAD
RESISTOR
(R
L )
INPUT VOLTAGE
(FROM MAIN
OUTPUT)
PART OF MAX 1800
ONA
AI
AI
ON
OFF
Q 1
C 1
C 2
C 3
D 1
D 2
D 3
C C
R 1
V OUT+
V OUT-
L
R 2
R C
MAIN
IN
COMP
DCON
PART OF
MAX1800
DL
FB
INPUT
1-CELL
LITHIUM-ION
Figure 10. Auxiliary Charge-Pump Configuration