Rainbow Electronics MAX1636 - Manual (Page 10)

IRC (1) 512-992-3377 (512) 992-7900
Detailed Description
The MAX1636 is a BiCMOS, switch-mode, power-sup-
ply controller designed primarily for buck-topology reg-
ulators in battery-powered applications where high
efficiency and low quiescent supply current are critical.
Light-load efficiency is enhanced by automatic Idle
Mode operation, a variable-frequency, pulse-skipping
mode that reduces transition and gate-charge losses.
The step-down, power-switching circuit consists of two
N-channel MOSFETs, a rectifier, and an LC output filter.
The output voltage is the average AC voltage at the
switching node, which is regulated by changing the
duty cycle of the MOSFET switches. The gate-drive sig-
nal to the N-channel high-side MOSFET, which must
exceed the battery voltage, is provided by a flying-
capacitor boost circuit that uses a 100nF capacitor
between BST and LX. The MAX1636 contains 10 major
circuit blocks (Figure 2).
The pulse-width-modulation (PWM) controller consists
of a Dual Mode™ feedback network and multiplexer, a
multi-input PWM comparator, high-side and low-side
gate drivers, and logic. The MAX1636 contains fault-
protection circuits that monitor the PWM output for
undervoltage and overvoltage. Bias generator blocks
include the 5V (VL) linear regulator and the 1.1V preci-
sion reference. The PWM uses a 200kHz/300kHz syn-
chronizable oscillator. The circuit blocks are powered
from an internal IC power rail that receives power from
either VL or V
CC . The synchronous-switch gate driver is
powered directly from VL, while the high-side-switch
gate driver is powered indirectly from VL via an external
diode-capacitor boost circuit.
MAX1636
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
10 ______________________________________________________________________________________
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Table 2. Component Suppliers
Figure 1. Standard Application Circuit
MAX1636
V+
0.1 µF
POWER INPUT
0.1 µF
1 µF
1nF
C1
Q1
CMPSH-3
Q2
C2
L1
*
R1
R2
R3
OUTPUT
4.7 µF
SHDN
*SEE RECTIFIER CLAMP DIODE SECTION
SKIP
DL
PGND
LX
DH
BST
OVP
VL
V
CC
SYNC
CSH
CSL
FB
10k
V CC
CC
GND
GND
RESET
REF
(847) 696-2000 (1) 847-696-9278
Marcon/United
Chemi-Con
Dual Mode is a trademark of Maxim Integrated Products.