DN505 - Dual Controller Provides 2μs Step Response and 92% Efficiency for 1.5V Rails

Dual Controller Provides 2µs Step Response and 92% Efficiency
for 1.5V Rails
Design Note 505
Mike Shriver
Introduction
The LTC®3838 is a dual output, dual phase buck controller
that employs a controlled constant on-time, valley current
mode architecture to provide fast load step response,
high switching frequency and low duty cycle capability.
The switching frequency range is 200kHz to 2MHz—its
phase-locked loop keeps the frequency constant during
steady-state operation and can be synchronized to an
external clock. The LTC3838 accepts a wide input range,
4.5V to 38V, and can produce 0.6V to 5.5V outputs.
VIN
4.5V TO 14V
+
CIN1
180μF
CIN2
22μF
w4
The remotely sensed VOUT1 has a voltage regulation accuracy of 0.67%, from 0°C to 85°C, even with a voltage
difference of ±0.5V between local ground and remote
ground. The current sense comparators are designed to
sense the inductor current with either a sense resistor
for high accuracy or with the inductor DCR directly for
reduced power losses and circuit size.
L, LT, LTC, LTM, Linear Technology, the Linear logo and PolyPhase are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
2.2Ω
LTC3838
1μF
VIN
SENSE1–
SENSE2–
SENSE1+
SENSE2+
BOOST1
BOOST2
0.22μF
7.5k
0.22μF
0.1μF
0.1μF
2.21k
L1
0.4μH
VOUT1
1.2V
25A
MT1
+
COUT2
330μF
w3
TG1
TG2
SW1
SW2
2.21k
MT2
DB1
DB2
2.2Ω
COUT1
100μF
w2
7.5k
DRVCC1
INTVCC
1μF
DRVCC2
EXTVCC
4.7μF
BG1
MB1
BG2
MB2
L2
0.4μH
VOUT2
1.5V
25A
COUT3 +
330μF
w3
COUT4
100μF
w2
PGND
10k
VOUTSENSE1+
15k
VFB2
10k
100k
PGOOD1
0.01μF
PGOOD1
PGOOD2
470pF
137k
ITH1
DTR1
VRNG1
RT
SGND
RUN1
ITH2
100k
PGOOD2
0.01μF
TRACK/SS1 TRACK/SS2
47pF
18.2k
10k
VOUTSENSE1–
220pF
DTR2
VRNG2
PHASMD
MODE/PLLIN
CLKOUT
RUN2
CIN1: SANYO 16SVP180MX
CIN2: MURATA GRM32ER61C226K
COUT2, COUT3: SANYO 2R5TPE330M9
COUT1, COUT4: MURATA GRM31CR60J107ME39L
DB1, DB2: CENTRAL CMDSH-3
L1, L2: VISHAY IHLP5050FDER0R4M01
MT1, MT2: INFINEON BSC050NE2LS
MB1, MB2: INFINEON BSC010NE2LS
47pF
40.2k
DN505 F01
300kHz
Figure 1. Dual Output, 1.5V/25A and 1.2V/25A Buck Converter Operating at FSW = 300kHz
08/12/505
1.5V/25A and 1.2V/25A Buck Converter
Figure 1 shows a dual 25A output buck converter synchronized to an external 300kHz clock. The controlled
constant on-time valley current mode architecture allows
the switch node pulses to temporarily compress when a
5A to 25A load step is applied to the 1.2V rail, resulting
in a voltage undershoot of only 58mV (see Figure 2).
The full load efficiency for the 1.5V and 1.2V rails is
91.8% and 90.8%, respectively, as shown in Figure 3.
The high efficiency is realized by the strong gate drivers,
optimized dead time and DCR sensing.
VO(AC)
50mV/DIV
58mV
VSW
10V/DIV
IL
20A/DIV
25A
LOAD STEP
20A/DIV
5A
5μs/DIV
DN505 F02
Figure 2. 20% to 100% Step Load Response of the
1.2V Rail at VIN = 12V, FSW = 300kHz, Mode = FCM
Detect Transient Feature Further Speeds Up
Transient Response
An innovative feature of the LTC3838 is the load release
transient detect feature. The DTR pin indirectly monitors the output voltage by looking at the AC-coupled
ITH signal. If the inferred overshoot exceeds a user
set value, the bottom FET turns off. This allows the
inductor current to slew down at a faster rate, which
in turn reduces the overshoot. Per Figure 4, a 32%
reduction in the overshoot is realized on the 1.2V rail.
Greater improvements occur at lower output voltages.
Conclusion
The LTC3838 is a dual output buck controller ideal for
applications that require a fast load step response, high
switching frequency, high efficiency and accurate output
voltages. Other features include selectable operating
modes: forced continuous mode (FCM) for fixed frequency operation or discontinuous mode (DCM) for
higher efficiency at light load, programmable current
limit thresholds, soft-start, rail tracking and individual
PGOOD and RUN pins. The LTC3838 comes in a 5mm
× 7mm QFN package or a thermally enhanced 38-lead
TSSOP package.
LTC3838
INTVCC
6
95
1.5V
90
470pF
3
80
1.2V
2
75
POWER LOSS (W)
4
1.5V
POWER LOSS
70
65
0
5
34.8k
DTR1
47pF
EFFICIENCY
85
EFFICIENCY
ITH1
5
1.2V
10
15
20
LOAD CURRENT (A)
VIN = 12V
1
FSW = 300kHz
MODE = CCM
0
25
30
DN505 F03
Figure 3. Efficiency and Power Loss of the 1.5V/25A
and 1.2V/25A Converter
The two channels operate 180° out-of-phase, which
permits the use of fewer input capacitors due to input
capacitor ripple current cancellation. For higher current
applications, two or more phases can be tied together to
form a single output, PolyPhase® converter. The benefits
include a faster load step response, reduced input and
output capacitance and reduced thermal dissipation.
Data Sheet Download
www.linear.com
DN505 F04a
38.3k
Figure 4a. Implementation of the Detect Transient
Feature on the 1.2V Rail
90mV
VO(AC)
100mV/DIV
WITH
DTR
IL
50A/DIV
VO(AC)
100mV/DIV
119mV
ORIGINAL
CIRCUIT
IL
50A/DIV
5μs/DIV
DN505 F04
Figure 4b. 100% to 20% Step Load Response of the
1.2V Rail with and without the Detect Transient Feature,
VIN = 12V, FSW = 300kHz, Mode = FCM
For applications help,
call (408) 432-1900, Ext. 3720
Linear Technology Corporation
dn505f LT/AP 0812 196K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2012
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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